Abstract A new, general model of decay of organic matter assumes that the various constituents of the decaying mass have (1) different rates of decomposition and (2) different rates of transformation to more or less decomposable forms. Existing models of the decay process are special cases of the general model that share three assumptions: (a) no transformation of detrital components, (b) equal rates of decay for one or two homogeneous constituents of the decaying mass, and (6) a uniform distribution of each type of constituent at the beginning of decay. A solution of the general model that relaxes these three assumptions fits previously published data closely. The general model unites existing models, permits comparisons of their assumptions, and provides a theoretical framework for developing and evaluating particular models that embody assumptions more in harmony with empirical knowledge of decay mechanisms.