Black liquor from the oxygen bleaching of soda pulp from wheat straw pre-treated with the enzyme cartazyme was characterized. A central composite design was used to examine the influence of oxygen bleaching variables (temperature (T), time (t), and soda concentration (S)) on some properties of the black liquor (pH, chemical oxygen demand (COD), total carbon (TC), total organic carbon (TOC), total solids (TS), total volatile solids (TVS), total mineral solids (TMS), and colour). Equations that relate the dependent and to the independent variables were derived that predict the former with errors < 20%. The least polluting black liquor (pH 7.86, COD 2248 mg/l, TC 671 mg/l, TOC 644 mg/l, TS 2609) mg/l, TVS 2188 mg/l, TMS 884 mg/l, colour 1151 colour units) was obtained by using a low T (90 degrees C), a low S (0.5%) and a short t (15 min) in the oxygen bleaching process. Bleaching under these conditions resulted in pulp and paper sheets the brightness, stretch and double fold number of which were only 10, 13 and 6%, respectively, worse than those obtained under the optimum operating conditions (viz. 90 degrees C, 1.5% soda and 15 min). On the other hand, the viscosity, breaking length and burst index were essentially the same and the yield and tear index were 3 and 2% higher, respectively, than under the optimum conditions. (C) 2000 Elsevier Science Ltd. All rights reserved.