In light of the predictions of two competing approaches to adult L2 acquisition – Full Access (FA) (e.g., White 1989, 2003; Schwartz & Sprouse 1996) and Failed Features (FF) (e.g., Hawkins & Chan 1997; Liceras & Díaz, 1999) – the present study examines the acquisition of inflected infinitives by English and Spanish/English bilingual adult learners of L2 Portuguese. Target-like acquisition of inflected infinitives requires the resetting of both a syntactic parameter (the Null Subject Parameter) and a morphological parameter (the Infl-parameter) for these learners. Since FF approaches maintain a post-critical period failure to acquire new L2 features lacking from the L1, target-like acquisition is predicted to not be possible. Conversely, FA approaches, which maintain adult parameter resetting is possible, predict that native-like competence of inflected infinitives is attainable, but not inevitably so. The data we present support FA approaches, demonstrating that advanced adult learners achieve native-like interpretative knowledge of Portuguese inflected infinitives. We also consider the role of L1 transfer and its possible implications, as they differ for both groups.