„Gosio“, „Wetterhahn“, „Minamata“, and „Black Foot Disease“ are only some synonyms counting for the hazards and risks of methylated metal(loid) organic compounds. Furthermore they point out how significant the properties of inorganic metal(loid)s differ from those of their methylated organic form. Thus, this dissertation deals with the speciation and toxicity of metal(loid)organic compounds with a particular focus on Hg, As and Sb in biological and environmental relevant systems. At least since the tragic accident of Prof. K. Wetterhahn as the result of dimethylmercury poisoning caused by the spill of a few drops of this chemical on her glove-covered hand, it is known that these compound can penetrate Latex gloves. However, only very little is known about permeation through other disposable glove materials or for different metal(loid) organic compounds like e.g. tetramethyl tin (Me4Sn) or trimethyl bismut (Me3Bi)) under typical laboratory conditions. With a new developed, heated permeation cell the breakthrough time of Me2Hg, Me4Sn, Me3GeCl, Me2AsI, and Me3Bi for various protective glove materials are determined. The results have shown, that most typically used disposable protective gloves (vinyl, vatex, nitrile) are penetrated within seconds and thus do not provide suitable protection. Methylated metal(loid) compounds are formed in the environment, favored by increased temperatures and anoxic conditions. In times of increasingly scarce of fossil fuel, biogas plants, which already have a great share in the production of renewable energy in Germany, are upcoming new diffuse and usually not accounted for sources of metal(loid) organic compounds (HotSpots). By the way, these plants are typically canvas-covered and metal(loid)s may also penetrate these foils. Therefore in this work four different biogas plants (biodigestor, postfermenter, condensation water and biogas) are analyzed. In all biogas samples permethylated metal(loid) organic compounds are detected (31-694 ng/m³ Me3As, 0-2 ng/m³ Me2Hg, 0-7000 ng/m³ Me3Sb und 0-79 ng/m³ Me4Sn). As expected both, the biodigestor as well as the postfermenter, are an origin of many partly and per methylated arsenic, antimony, mercury, and tin species. The gas condensates are estimated as a potential sink for methylated metal(loid) compounds. Beside highly toxic dimethyl mercury (> 0.1 µg/L) different methylated arsenic species (e.g. Me3As > 90 µg/L) as well as several not identified antimony species were detected in the condensates. While the toxicology of the inorganic form of the metal(loid) is mostly well documented and some of them are relatively low toxic, the toxicity of their metal(loid) organic species is mostly hard to determine. However, the permeation capability of metal(loid) organic compounds through membranes can also be used to evaluate the toxicity of these highly volatile substances. Therefore CHO-9 cells—an established cell system for toxicity testing, CaCo cells—human colon cells, and HepG2 cells cells—human hepatic cells are grown on a “ThinCert” membrane and exposed against different metal(loid) organic compounds in a new designed exposition system. The results showed that dimethylmercury was most toxic to all three used cell lines followed by dimethylarsenic iodide. Tetramethyltin was the least toxic compound; and the toxicity was also dependent upon the cell type. Human colon cells (CaCo) were most susceptible to the toxicity of the volatile compounds compared to the other cell lines. However, volatile metal(loid) compounds can be toxic to mammalian cells already at very low concentrations but the toxicity depends upon the metal(loid) species and the exposed cell type. Furthermore, the toxicity also depends on the uptake speed and the intracellular biotransformation. In particular for arsenic, MMA(III) (monomethylarsonous acid) is one of the most important metabolites. Based on previous experiences time-resolved biotransformation of MMA(III) in hepatoma cells and speciation in the cell residues where performed by HPLC/ICP-MS. As a result of the speciation analysis it can be concluded that MMA(III) is immediately taken up and rapidly bound to proteins and other cellular structures, first followed by the generally accepted methylation to DMA, and in a second step by the degradation of affected proteins and cellular structures in the lysosome. Because of the comparison of the relation of mono- und dimethylated arsenic in the lysates and the cell residues it can be assumed that the methylation takes place in the cells, the methylated species are staying in the cell and are only slowly excreted. Several parts of this dissertation have already been published in relevant scientific journals.