Drawing from the organizational learning, transactions costs and resource based theories, we argue that the larger the Alliance Portfolio Size (APS) of the partners, the lower the likelihood that the alliance will be expanded. We also argue for an interactive effect by proposing that high levels of experience will neutralize some (but not all) of the detrimental impact of a large APS on the likelihood of alliance expansion. We deploy the case-control methodology to select a sample of 182 alliances from the biotechnology and pharmaceutical industries spanning the period 1980–2004. Results from logistic regression analyses support our predictions.