ECONOMISTS have long agreed that the rate of interest on a loan depends on the risks the lender incurs. But how lenders estimate these risks has been left largely to conjecture. This paper presents and tests a hypothesis about the determinants of risk premiums on corporate bonds. By risk premium is meant the difference between the market yield on a bond and the corresponding pure rate of interest. My hypothesis is as follows: (1) The average risk premium on a firm's bonds depends first on the risk that the firm will default on its bonds and second on their marketability. (2) The "risk of default" can be estimated by a function of three variables: the coefficient of variation of the firm's net income over the last