In this article, we argue that insights concerning the word-based nature of morphology, especially the hypothesis that periphrastic expressions are cross-linguistically common exponents of lexical relations, permit a novel lexical constructional analysis of periphrastic predicates that preserves the restriction of morphosyntactic mapping operations, such as passive, to the lexicon. We do this in the context of the periphrastic Thai thuuk passive, justifying in detail the monoclausal status of the construction, its flat phrase structure, the semantics of affectedness associated with it, and its paradigmatic opposition with other passive constructions in the language. Building on the proposal of Bonami & Webelhuth (2013) and Bonami (2015) that a periphrase relies on a form of the main verb that selects collocationally for an auxiliary element, we develop an analysis of Thai periphrastic passives in which the surface syntax of these predicates is mediated by appropriate lexical representations. Crucially, the rearrangement of arguments in the passive is done lexically, via lexical rule, rather than in the syntax. The resulting analysis is consistent with the classical tradition of Word and Paradigm morphology, which posits periphrastic expression as one of several encoding strategies for the realization of morphosyntactic information within words.