This paper investigates firms’ abilities to tacitly collude when they each monopolize a proprietary aftermarket. When firms’ aftermarkets are completely isolated from foremarket competition, they cannot tacitly collude more easily than single-product firms. However, when their aftermarket power is contested by foremarket competition as equipment owners view new equipment as a substitute for their incumbent firm’s aftermarket product, profitable tacit collusion is sustainable among a larger number of firms. Conditions under which introduction of aftermarket competition hinders firms’ ability to tacitly collude are characterized.