This article investigates the decision process used by university presses when selecting or deselecting e-book vendors for academic libraries. It focuses on three research questions: (1) which vendors' university presses worked with; (2) how university presses used different vendors; and (3) why university presses worked or did not work with certain vendors. A series of one-on-one interviews were conducted with 19 participants from 18 different university presses in the US. Findings show participants use a core of vendors identified as the ‘Big Four’ plus several other ‘optional’ vendors. The reasons that encourage and discourage presses to work with certain vendors are investigated and reveal a mutual reinforcement cycle that explains the process by which the Big Four vendors became both the main distribution vendors for university presses and the main providers for academic libraries for e-books in humanities and social science. The research also discusses efforts made by several university presses and other vendors to challenge the dominance of the Big Four.