The passive and impersonal uses of the Spanish clitic se have been the focus of an important linguistic debate; they are often considered to be similar because they both de-emphasize the logical subject of the verb, but they differ in the way they affect the verb’s transitivity. Many theories have suggested that the clitic se raises to subject position and that these two forms are syntactically identical with a difference in the application of subject-verb agreement (e.g. Cinque 1988, Oesterreicher 1992, and Rivero 2002). Amaya Mendikoetxea’s (2008) analysis of Romance clitic impersonal constructions se/si offers a novel understanding, not only of the relationship between these two Spanish clitic constructions, but also of how the impersonal se construction differs from simple transitive sentences. Specifically, she theorizes that both impersonal and passive constructions containing se contain a generic null pronoun (which she calls G-pro) as Spec of vP and that they differ in whether or not v assigns accusative case to the verb’s complement. This difference in case marking determines whether the verb’s complement raises to subject (to yield the passive se construction) or whether the verb’s complement remains in place and a phonologically null expletive fills the subject position. With this analysis in mind, a CHILDES longitudinal study is conducted to assess children’s acquisition of Mendikoetxea’s proposed structures. The results confirm Mendikoetxea’s basic claim that there is a distinction among simple transitive sentences, impersonals with se and passives with se.