In two empirical studies, judgements that native speakers of German make about the decomposability of English idioms were investigated. A decomposable idiom is an idiom whose individual components contribute to its figurative meaning, whereas the constituents of a nondecomposable idiom do not make such a contribution. The findings were analysed and compared to native judgements. The Model of Dual Idiom Representation is introduced in order to explain the differences between the two groups. At the lexical level, the model postulates the parallel existence of idiom entries and constituent entries. The degree of decomposability and the frequency with which the idiom is encountered determine its lexical representation. If there is no idiom entry for a particular idiom, conceptual representations are accessed during comprehension. Because nonnative speakers encounter idioms less often than native speakers, the first language (L1) and second language (L2) lexicon vary with regard to the number of idiom entries.