Empirical studies and much marketplace opinion have it that the spread between private money market rates and the U.S. Treasury bill rate of comparable maturity is due to differential default risk, liquidity risk, and relative supplies. This paper presents an argument and empirical evidence that the bulk of the systematic and medium-term differential between privates rates—in this case, domestic CDs, commercial paper, bankers' acceptances, and Eurodollar CDs—and the T-bill rate is due to the exemption of interest on Treasury securities from state and local taxation. In the case of Eurodollar CDs, the additional and major systematic factor explaining the spread (vis a vis T-bills) is the exemption of Eurodollar CDs from the Federal Reserve's “tax” via reserve requirements. An empirical section confirms the role of standard default risk, liquidity risk, and market “absorption” variables in determining short-term deviations from tax-adjusted parity. The taxadjusted parity condition, however, remains the major systematic and medium-term determinant—certainly more important than has been suggested previously in the literature.