This experimental study on the acquisition of the double-object construction in Spanish as a second language (L2) by a group of first language (L1) English adults investigates the role of Universal Grammar (UG) and its interaction with L1 in two modules of grammar: morphosyntax and semantics. The double-object construction in Spanish differs from its English counterpart in its morphosyntactic properties (case, clitic doubling, word order) and its semantics (interpretation of arguments and restrictions on the construction). Results show that L2 learners are sensitive to most of the morphosyntactic properties of the double-object construction but lag behind in the acquisition of its semantics. The experimental group shows evidence of UG-constrained acquisition in their sensitivity to morphosyntactic properties not instantiated in their L1 as well as in their nontarget but UG-licit analysis of the semantic restrictions of Spanish double objects. The dissociation between level of knowledge of morphosyntax and of semantics suggests that modularity of grammar is reflected in SLA exactly as it is in L1 acquisition.