Firms increasingly adopt an open innovation model in which they rely on technology alliances to complement and supplement their internal innovation efforts. Although previous studies provide in-depth insight into the impact of technology alliances on the innovation performance, they remain relatively silent on how technology alliances eventually influence the financial performance of the firm. The purpose of this paper is to develop and test a conceptual framework that disentangles both the value-enhancing and cost-increasing effects of technology alliances on financial performance. The model was tested with a sample of 305 Belgian manufacturing firms. Combining data from the Belgian Community Information Survey (CIS IV) database and the BELFIRST database, structural equation analyses were conducted on the connection among technology alliance portfolio diversity, product innovation performance, and financial performance. This study's data provide empirical confirmation for the assumption of existing research that technology alliance portfolio diversity has an indirect positive impact on financial performance via increased product innovation performance. However, a direct cost-increasing effect of technology alliance portfolio diversity on financial performance is observed. Moreover, the structural equation analyses suggest that, in the short-term, the direct cost-increasing effect of technology alliance portfolio diversity exceeds the indirect value-generating effect of technology alliances. These findings contribute to the current research on open innovation in two important ways. First, these results support the open innovation model by illuminating the interconnectedness between internal and external innovation strategies. In particular, technology alliance portfolio diversity has a positive impact on internal innovation efforts, which increases product innovation performance. Second, the findings complement the focus of existing open innovation research on the value-generating properties of technology alliances, directing attention to the cost-increasing effects of such collaborative strategies. On a managerial level, these findings suggest that, when making technology alliance decisions, managers not only should consider the potential benefits of such collaborative strategies but also should take into account the additional costs of intensifying the technology alliance portfolio.