This paper develops a structural model that determines default spreads in a setting where the debt's collateral is endogenously determined by the borrower's investment choice, and a demand variable with permanent and temporary components. We also consider the possibility that the borrower cannot commit to taking the valuemaximizing investment choice, and may, in addition, be constrained in its ability to raise external capital. Based on a model calibrated to data on office buildings and commercial mortgages, we present numerical simulations that quantify the extent to which investment f lexibility, incentive problems, and credit constraints affect default spreads.STARTING WITH THE SEMINAL WORK of Black and Scholes (1973) and Merton (1974), researchers have developed contingent claims models to value risky debt. A subset of these models, known as structural models, assumes that markets are perfect and that the value of the collateral of the debt can be viewed as exogenous. 1 This approach to pricing debt is in sharp contrast to the theoretical * Titman and Tompaidis are with the McCombs School of Business, University of Texas at Austin.