We use data from in-depth interviews with business lobbyists in Brussels to investigate why they choose to join lobbying coalitions. We find that lobbyists face two competing institutional incentives. First, they are confronted with incentives to ally with other European organisations, develop multilateral policy messages, and communicate messages to the Commission and the Parliament. Simultaneously, they face inducements to join narrower coalitions, develop bilateral policy messages, and direct those messages at the Council.Lobbyists' receptivity to these incentives -and thus their choices of lobbying coalitionsdiffers with their age, educational background, and with the type and ownership structure of the organisations they represent. Combined, our findings contribute to the limited, mainly American literature on interest coalitions by demonstrating that lobbyists operate in complex institutional environments, and that their interpretations of and reactions to institutional complexity are shaped by individual-and organisational-level factors.