Multidivisional firms often fail to take advantage of innovations that involve combining resources from distinct divisions. This failure of cross-line-of-business innovation is a consequence of design choices employed to execute the firm's strategy: in organizing around its core businesses, the firm renders interdependence between divisions residual to the formal structure. As a result, those innovations which involve cross-line-of-business interdependence are trumped by the firm's articulated strategy and structure. Social structures could, potentially, fill this coordination gap. But social structures associated with the initiation of interdependent innovation are inversely associated with their execution. We build a dynamic, corporate-level, evolutionary model in which individuals autonomously initiate cross-line-of-business projects not through the formal structure of the firm, but using contacts from their own social networks. Some of these projects are selected and actively supported by senior executives; this support sends clear signals about what collaboration is valued by the firm, which gives other actors powerful, albeit informal, incentives to connect with others across the interunit boundary. As a result, the sparse interunit social structure that was conducive to initiation changes, becoming much more cohesive (at least locally) and is able to support execution and retain these interdependent innovations. Thus, where intra-divisional innovations are primarily driven by organizational structure, we suggest that interdivisional innovations are driven primarily by social networks. Copyright © 2007 Strategic Management Society.