A pyrolytic study of a variety of lignins prepared from hardwood and softwood species, and of selected commercial lignins, was performed using isothermal and dynamic thermogravimetric analysis and pyrolysis-gas chromatography. The lignins were evaluated according to weight loss, rate of weight loss, and products formed during the pyrolysis in an inert atmosphere. These data were then compared to nitrobenzene oxidation data. Infrared spectroscopy was used to determine structural changes of lignin during initial weight loss in the temperature range of 250° to 320°C. Structural changes appear to occur without any appreciable weight loss. Lignin's thermal behavior during pyrolysis depends upon the degree of condensation present in the initial lignin. Thermogravimetric analysis is shown to be a simple and accurate method to determine the extent of lignin condensation. The yield of monomeric phenols is also dependent on the degree of lignin condensation.