The literature on party group switching in the European Parliament contends that members re-affiliate primarily for strategic reasons. This article advances the discussion by also considering the occurrence of non-strategic switches which follow the collapse of weakly institutionalized groups. Using an original dataset which includes DW-Nominate scores (1979–2009), I operationalize policy-seeking behavior among strategic switchers by deriving member- and delegation-to-group policy distance variables. The pooled logistic regression models using a penalized maximum likelihood estimator make it possible to address quasicomplete separation, and the results show that members from large groups and delegations have significantly lower odds of switching. Further, as members or delegations become incongruent with their group, the odds of switching increase. The study has important implications for research investigating the relationship between weak party institutionalization and parliamentary behavior.