Pyrolysis-gas chromatography has been employed to study the transformation of organic matter during the composting process of ground barley straw and pear-tree wood. Some of the major fragments have been identified by gas chromatography-mass spectrometry, and the abundance of carbohydrate derivatives in the fresh materials and the amount of phenolic products in composted materials have been established. Several parameters, such as the diversity index and pyrolysis ratios, were used to study the changes in the pyrograms during the composting process. The diversityindex in wood shows little variation, decreasing slightly from the 400th day; straw pyrograms show increased diversity up to the 230th day, mostly owing to a steady reduction in the carbohydrate content. Some pyrolysis ratios can be used as control indices of the composting process. Most of them include carbohydrate derivatives in the numerator and aromatic or heterocyclic compounds in the denominator, such as furfural + cyclopentenone/pyrrole +2-methylpyrrole. A large increase in aromatic compounds was detected in the composted straw compared with fresh material, and a similar but smaller increase in a wood compost. In general, all pyrolysis products show lower variations in wood than in straw composting processes. © 1985.