The Cantonese ‘coverb’ construction, a serial verb construction in which the first verb (the ‘coverb’) has a preposition-like meaning and function, presents a challenge for theories of wh- dependencies and island constraints.  Coverbs resist extraction of their objects by topicalization or relativization, a fact which has often been explained in terms of a preposition-stranding constraint in accounts of similar facts in Mandarin.   However, Cantonese coverbs display the morphosyntactic properties of verbs, suggesting that they cannot be prepositions.  In this paper, we propose that coverbs are verbs, and that the relevant extraction constraint is a kind of adjunct island constraint.  This proposal is supported with experimental evidence from a sentence judgment task.  Two key findings were as follows: (1) listeners judged extraction from a coverb phrase as significantly less acceptable than extraction from a simple clause; (2) listeners judged sentences both with and without aspectual marking (verbal morphosyntax) on the coverb as highly acceptable.  Together, these findings support our proposal that coverbs are verbs (not prepositions) and that coverb phrases form a kind of adjunct island.  However, we show that existing adjunct island conditions (such as the CED) are not adequate to account for our data.  Following John A. Hawkins’ (1999) processing-based theory of filler-gap dependencies, we propose a simple, language-specific formulation of the extraction constraint, and we argue that this constraint is more generally motivated by a processing principle called Avoid Competing Subcategorizors—one of the same principles that motivates preposition-stranding constraints in other languages.  Thus, although object extraction is prohibited by a kind of adjunct island constraint, the function of the constraint in processing efficiency is similar to that of a preposition-stranding constraint.