A model of the tax structure of interest rates is developed and simple approximate expressions relating yield to coupon are derived. The effect on these simple expressions of alternative assumptions about holding period length, expectations of future interest rates, and other factors, is evaluated. It is shown that with recent U.S. yield averages the new-seasoned yield spread varies with the new-seasoned coupon spread as the theory prescribes. It is concluded that new issue yield averages should provide a more reliable measure of the cost of debt capital than is provided by seasoned yield averages.