This chapter puts forward an account of auxiliary selection in terms of inflexional classes (following, e.g., Bonami 2015:97; Baerman, Brown, and Corbett 2017:28f.), theoretically couched within Stump’s notions of content paradigm, form paradigm, and segregated inflexional classes (Stump 2016). It is argued that the inflexion-class behaviour of auxiliary selection can best be seen in cases of mixed perfective auxiliation systems attested in a range of Italo-Romance varieties, where an intricate alternation of the two auxiliaries HAVE and BE is found within one and the same paradigm. The chapter focuses on cases where one coherent class of lexemes, namely reflexives, is split between the two auxiliaries giving rise to a mixed paradigm which is an instance of heteroclisis (cf. Stump 2006; Kaye 2015; Bach 2019). It is hypothesized that this kind of reflexive heteroclisis might be one of the origins of mixed perfective auxiliation systems in Italo-Romance.