This research studies the evolution of the composition of an alliance portfolio from a coopetition perspective. Building on resource dependence theory, market uncertainty appears to be a driver of alliance portfolio formation and evolution. Scholars have previously neglected key dimensions in analyzing the composition of firms' alliance portfolios: the partner type (pure partner or competitor) and partner interactions (horizontal, vertical or mixed). We build on the coopetition and alliance portfolio literature to explore (1) the composition of an alliance portfolio and (2) its evolution over time. We illustrate our theoretical framework with a longitudinal single-case study of Air France's alliance portfolio. First, we show that when market uncertainty is high, firms do not increase their reliance on collective strategies, but they do modify the composition of their portfolio. Second, to address high levels of market uncertainty, firms rely more on coopetitive alliances than on collaborative alliances. Third, firms use more horizontal than vertical interactions when market uncertainty is high.