A normalized design was used to examine the influence of independent variables (alcohol concentration, cooking time and temperature) in the catalytic soda-ethanol pulping of rice straw on various mechanical properties (breaking length, burst, tear index and folding endurance) of paper sheets obtained from each pulping process. An equation of each dependent variable as a function of cooking variables (independent variables) was obtained by multiple non-linear regression using the least square method by MATLAB software for developing of empirical models. The ranges of alcohol concentration, cooking time and temperature were 40-65% (w/w), 150-180 min and 195-210degreesC, respectively. Three-dimensional graphs of dependent variables were also plotted versus independent variables. The optimum values of breaking length, burst and tear index and folding endurance were 4683.7 (m), 30.99 (kN/g), 376.93 (mN m(2)/g) and 27.31, respectively. However, short cooking time (150 min), high ethanol concentration (65%) and high temperature (210degreesC) could be used to produce papers with suitable burst and tear index. However, for papers with best breaking length and folding endurance low temperature (195degreesC) was desirable. Differences between optimum values of dependent variables obtained by normalized design and experimental data were less than 20%. (C) 2003 Elsevier Ltd. All rights reserved.