We use a detailed dataset on electricity transactions to investigate changes in market structure following the deregulation of the electricity sector, as well as the consequences for prices. We show that deregulation was effectively delayed by intermediate degrees of vertical integration, such as long-term contracts and common ownership. To account for these mechanisms, we look at the impact of effective deregulation: the portion of a market served by companies not related to the incumbent utility. We find that effective deregulation occurs several years after the initial vertical separation of incumbent utilities, and, when it occurs, prices increase. We examine wholesale market transactions in more detail using contracts for a subset of our data. We find that higher prices appear to be driven by purchases between affiliated companies, pointing to one mechanism by which an intermediate form of vertical integration plays a role.