In this article, we introduce the effect of “constructional contamination”. In constructional contamination, a subset of the instances of a target construction deviate in their realization, due to a superficial resemblance they share with instances of a contaminating construction. We claim that this contaminating effect bears testimony to the hypothesis that language users do not always execute a full parse while interpreting and producing sentences. Instead, they may rely on what has been called “shallow parsing”, i. e., chunking the utterances into large, unanalyzed exemplars that may extend across constituent borders. We propose several measures to quantify constructional contamination in corpus data. To evaluate these measures, the Dutch partitive genitive is taken under scrutiny as a target construction of constructional contamination. In this case study, it is shown that neighboring constructions play a crucial role in determining the presence or absence of the -s suffix among instances of the partitive genitive. The different measures themselves, however, are not construction-specific, and can readily be used to track constructional contamination in other case studies as well.