One of the defining properties of Marshallese (Austronesian, Oceanic, Micronesian) long passives is that they allow one of two prepositions in what roughly corresponds to English by phrases. These prepositions are ippa:n and in. Although the semantics of these two prepositions appears to be identical in passives (i.e. both introduce an agent phrase), there are syntactic differences between sentences containing these prepositions, a fact which leads Willson (2008) to conclude that ippa:n and the agent phrase form a constituent, while in and the agent phrase do not. This conclusion presents a problem for the traditional analysis of passives because the traditional analysis offers no explanation as to why one preposition+agent string is a constituent but the other is not. In order to explain the differences in the syntax of these strings, I propose an analysis of passives reminiscent of Collins’ (2005) smuggling approach to English passives and argue (1) that in, like the English by, is the head of a Voice projection and (2) that the agent DP in passive sentences containing in is merged as the specifier of vP and receives an agent theta role from the verb. A similar analysis cannot be proposed for sentences containing ippa:n, however, as such an analysis would predict that ippān and the agent are not a constituent. Instead, I argue that ippa:n+agent strings are adjunct prepositional phrases, a fact that can explain why these strings can appear in nonpassive sentences, as well as the constituency differences between long passives containing in and those containing ippa:n.