Spanish Differential Object Marking (DOM) is a phenomenon in which some direct objects are marked with the dative preposition a, depending on semantic properties of the object, verb, and subject, including animacy and specificity of the object. This paper presents an experimental study of the acquisition of DOM by native English speakers who are advanced L2 learners of Spanish living in Spain. The goal of the study is to shed light on competing theories of L2 acquisition, particularly the Interpretability Hypothesis (Hawkins and Hattori 2006) and the Feature Reassembly Hypothesis (Lardiere 2008, 2009). Participants were found to have acquired the animacy condition more successfully than the specificity condition, which is argued to support the Feature Reassembly Hypothesis, given Torrego's (1998) syntactic analysis of DOM. It is also suggested that the pattern of observed results can be explained in terms of the Interface Hypothesis (Tsimpli and Sorace 2006), under the assumption that the specificity requirement implicates the syntax-discourse interface, while the animacy requirement only implicates the syntax-semantics interface.