The effect of two decomposition processes, digestion and composting, was compared on the quality of the final residue from two substrates: hay and straw. The digestion was done by using dwarf goats on digestion crates and the composting process was simulated in buckets. Composting was allowed to continue to the same level of organic matter loss as achieved in a digestibility trial. The results indicated that the pattern of biomass decomposition, as measured by the fibre analysis in the Goering and Van Soest method, was similar between both processes and substrates. Absolute amounts of acid detergent lignin (ADL) did not change significantly throughout both decomposition processes, but the quantity of other fibre components decreased. Chemically, the ADL fraction made up a large part of the humus in the soil and though the results were variable, they suggested that the initial ADL content of a substrate indicates its humus building capacity. Concerning the nitrogen fractions, however, there were larger differences between the decomposition processes and substrates. Digestion left less nitrogen in the organic matter than composting, and the composting of straw even resulted in a net increase in nitrogen content.