It has been proposed that external interfaces are vulnerable to residue optionality, whereas pure syntax and internal interfaces are acquirable in second language (L2) acquisition (Sorace, 2005, 2011; Sorace and Filiaci, 2006). The proposal was tested in this article through the interpretation of overt and null embedded arguments in L2 Chinese grammars. The article identifies two types of null elements in Chinese: null element[subscript ziji] and null element[subscript topic]. Null element[subscript ziji] is a purely syntactic category, whereas null element[subscript topic] is a syntax-discourse interface category. Being a D-pronoun, "ta" "he/him" can refer to either the matrix subject or a discourse entity in the embedded argument position. Its interpretation involves lexicon-syntax and syntax-semantics internal interfaces. Results from a picture judgment task showed that external interfaces were acquired as well as pure syntax and internal interfaces, which supports the claims of Ivanov, 2009; Iverson et al., 2008; Kras, 2008; Rothman, 2007, 2009; Slabakova and Ivanov, 2011. In addition, null element[subscript topic] was acquired at different states in different sentence positions. This supports the claim that interface categories should not be considered holistically (Yuan, 2010). The article further speculates that cross-linguistic influence and the nature of the possible positive evidence may have contributed to the difference here in terms of representation and/or processing. (Contains 6 tables, 2 figures, and 2 notes.)