Complete carbon-13 nuclear magnetic resonance assignments have been made for 22 4H-3,1-benzoxazin-4-ones. These compounds are alternate substrate inhibitors of human leukocyte elastase, a serine protease involved in tissue degradation. Correlations between the carbon chemical shifts and rates of alkaline hydrolysis are consistent with hydrolysis via attack at C4, and are also useful in the selection of parameters for structure–activity analysis.