In recent second language (L2) research it has been proposed that unlike linguistic phenomena pertaining to internal interfaces, those at external interfaces pose greater difficulty to adult L2 learners and may not be fully acquired. It has further been pointed out that such problematic acquisition at the interface level should not be attributed to the entire interface and requires a more nuanced examination, and this is what the current article aims to provide. An empirical study reported here investigates whether Russian-speaking learners are able to acquire Chinese "wh"-topicalization that lies at the syntax-discourse interface, an instance of the external interface. The results indicate that although very advanced Russian speakers can acquire "wh"-topicalization in their L2 Chinese, the ability of "wh"-elements to topicalize in Russian-Chinese interlanguage grammars seems to be determined by an internal structure underlying Chinese wh-elements, and this is likely to be a variable that affects the linguistic behaviour at the interface level in the L2.