Six lexical decision experiments were conducted to examine the influence of complex structure on the processing speed of English compounds. All experiments revealed that semantically transparent compounds (e.g., "rosebud") were processed more quickly than matched monomorphemic words (e.g., "giraffe"). Opaque compounds (e.g., "hogwash") were also processed more quickly than monomorphemic words. However, when the experimental materials and/or procedure encouraged decomposition/integration, this advantage disappeared. This research suggests that morphological decomposition initiated by the existence of complex structure results in the availability of both the lexical and semantic representations of compound constituents, regardless of whether the compounds are transparent or opaque, and that meaning composition is attempted. This meaning composition further speeds up transparent compound processing beyond lexical facilitation but slows down opaque compound processing because the computed meaning for opaque compounds conflicts with the retrieved meaning. (Contains 7 tables.)