The present study examines the acquisition of double object constructions (DOCs) ("Susan gave Peter an apple") by 90 Basque/Spanish learners of English as a third language (L3). The aim of this study was to explore whether (i) learners established a distinction when accepting DOCs vs. prepositional phrase constructions (PPCs) ("Susan gave an apple to Peter"), (ii) they were sensitive to the morphological and semantic constraints found in English DOCs and (iii) proficiency effects could be observed. Results from a self-paced reading task and an auto-paced reading task showed that Basque/Spanish learners were more accurate in morphosyntactic properties than in semantic ones. Even though learners showed difficulties in both morphological and semantic constraints, these seem to be overcome by increasing proficiency.