We develop new liquidity measures for bond markets. Existing measures suffer from the combination of two effects. First, transaction costs in OTC markets strongly depend on trade size. Second, many bonds trade only scarcely with strongly differing trading volumes. Therefore, changes in average transaction costs often indicate changing trade sizes rather than changing liquidity. We combine full-sample information for the size-cost relation with individual transaction data to eliminate such measurement problems. We find that size-adapted measures make a difference when analyzing liquidity dynamics in the U.S. corporate bond market, liquidity differences between bonds, and the asset pricing implications of liquidity.