This study examines the placement of an adverb with respect to a modal or perfect auxiliary in English (e.g., It might potentially escape / It potentially might escape). The data are drawn from two large, socially stratified corpora of vernacular English (Toronto, Canada, and York, England) and thus allow a cross-dialect perspective on linguistic and social correlates. Using quantitative sociolinguistic methods, I demonstrate similarity in the varieties, with the postauxiliary position generally strongly favored. Of particular importance is the structure of the auxiliary phrase; when a modal is followed by the perfect auxiliary (e.g., It might have escaped), the rates of preauxiliary adverb placement are considerably higher. As the variation is chiefly correlated with linguistic, rather than social factors, I apply recent proposals from Generative syntax to further understand the grammar of the phenomenon. However, the evidence suggests that the variability seen here is a result of postsyntactic, rather than syntactic, processes.