This paper attempts to formalize the transaction cost theory of the firm. Building on the formal approach of Grossman and Hart (1986), a model is developed to capture the essential elements of the transaction cost theory, particularly those that are distinct from the formal property rights theory (PRT). In contrast to the PRT model, ours focuses on specific investments in alienable assets and ex post transactional inefficiencies. We define integration of two firms to imply common ownership of alienable assets from both firms, which entails control rights over the use of the assets as well as claims on their residual value. One important advantage of the model is its ability to deal with integration between non-owner-managed firms.