This article explores the distribution of morphological case in Spanish nominalizations and shows that there is a connection between morphological case and event structure. Most nominals govern genitive case de on their internal arguments but some allow or require a different morpheme, a, reminiscent of differential object marking. I argue that the event structure of the nominalization is the crucial factor in choice of a, inasmuch as a is limited to process nominals that do not entail a change of state. The same distinction between process and change-of-state nominals is then extended to two other empirical puzzles regarding the interpretation of genitive arguments in nominalizations. I present a formal analysis assuming a syntax of events inspired by Ramchand (2008).