Colorimetry gave consistently lower uronic anhydride values for four species of wood than did measurement by evolved CO2. These differences were partly related to CO2evolution from nonuronide wood extractives, and partly to CO2from nonuronide carbohydrates or lignin. After the usual benzene-ethanol extraction to remove interfering extractives, the uronic anhydride contents by gravimetric CO2analyses were still 30 to 40% higher than those by colorimetry. Based upon the greater selectivity of the colorimetric method, the authors conclude that colorimetry provides the more accurate uronic anhydride analyses of natural materials.