The study is concerned with the relative synchronic stability of three contrastive sibilant fricatives /s (sic)/ in Polish. Tongue movement data were collected from nine first-language Polish speakers producing symmetrical real and non-word CVCV sequences in three vowel contexts. A Gaussian model was used to classify the sibilants from spectral information in the noise and from formant frequencies at vowel onset. The physiological analysis showed an almost complete separation between /s (sic)/ on tongue-tip parameters. The acoustic analysis showed that the greater energy at higher frequencies distinguished /s/ in the fricative noise from the other two sibilant categories. The most salient information at vowel onset was for /(sic)/, which also had a strong palatalizing effect on the following vowel. Whereas either the noise or vowel onset was largely sufficient for the identification of /s (sic)/ respectively, both sets of cues were necessary to separate /(sic)/ from /s (sic)/. The greater synchronic instability of /(sic)/ may derive from its high articulatory complexity coupled with its comparatively low acoustic salience. The data also suggest that the relatively late stage of /(sic)/ acquisition by children may come about because of the weak acoustic information in the vowel for its distinction from /s/.