Long-term decomposition data are presented for several types of foliar and fine root litter in different stands in Wisconsin and Massachusetts, U.S.A. Changes in mass remaining as well as nitrogen and carbon fraction (extractives, cellulose, lignin) concentration are reported. Three models were developed for describing change in mass remaining with time: a litter-specific exponential decay function (statistical fit of data for each litter type), a generalized exponential decay function (k predicted from initial litter chemistry), and a carbon fraction model that calculates the weight loss of each carbon fraction individually as a function of current carbon chemistry regardless of litter type. The exponential decay function fits all litter data well for the portion of decomposition described here, but would not be appropriate for modeling longer term decomposition. Both the generalized and carbon fraction models predicted weight loss accurately. All litter types had similar carbon fraction chemistries at the end of the first phase of decomposition described here and also exhibited a narrow range of changes in nitrogen concentration per unit weight loss. It is concluded that the length of time required to convert litter into soil organic matter and the chemistry of the material produced by this process can be predicted from initial litter chemistry and (or) relatively short-term litter decay data. Key words: immobilization, mineralization, humus, lignin, cellulose, extractives.