A central composite experimental design was used to investigate the influence of incubation conditions (vinasse added and operation time) for vinasse/cotton waste mixtures on the properties of products obtained (pH, electrical conductivity, organic matter, Kjeldahl-N, C/N ratio, biodegradability, Kjeldahl-N losses and germination index) in order to determine the best in-vessel composting conditions. The range of the independent variable measured was 0-80% for vinasse added and 1-50 days for operation time. A second-order polynomial model consisting of two independent process variables was found to accurately describe the vinasse/cotton waste incubation. The differences between the experimental values and those estimated by using the equations never exceeded 10% of the former. Obtaining products with acceptable chemical properties, high biodegradability and minimum N losses entails operating at medium operation time (20-35 days) and 20-30% of vinasse added. (C) 2002 Elsevier Science B.V. All rights reserved.