A central composite design was used to investigate the influence of the cooking conditions (time, temperature and phenol concentration) for wheat straw with phenol-water mixtures on the properties of the pulp obtained (yield and holocellulose, alpha-cellulose, lignin and ethanol-benzene extractable contents) and the pH of the resulting wastewater. A second-order polynomial model consisting of three independent process variables was found to accurately describe the organosolv pulping of wheat straw. The equations derived predict the yield, the holocellulose, alpha-cellulose, lignin and ethanol-benzene extractable contents of the pulp, and the pH of the wastewater with multiple-R, R-2 and adjusted-R-2 high vales. The process variables must be set at low variables in order to ensure a high yield and pH. Conversely, if high holocellulose and alpha-cellulose contents, and low lignin and ethanol-benzene extractable contents are desired, then a high temperature (200 degrees C), long cooking time (120 min), and intermediate phenol concentration (65%) must be used. (C) 1997 Elsevier Science Ltd.