I introduce a favor exchange model where favors are substitutable and study bilateral enforcement of cooperation. Without substitutability, the value of a relationship does not depend on the rest of the network, and in equilibrium there is either no cooperation or universal cooperation. When favors are substitutable, each additional relationship is less valuable than the previous, and intermediate levels of cooperation are observed. I extend the model to allow for transfers, heterogeneous players, and multilateral enforcement. My results can explain the stratification of social networks in post-Soviet states and the adoption of different enforcement mechanisms by different groups of medieval traders.