Sugar cane press mud prettied by liquid hot water was separated into a,liquid fraction and a solid fraction. These fractions were blended With vinasse in different proportions, according to a three factor mixture design, to evaluate their synergetic and antagonistic effects. on the methane yield. The biochemical methane potential was determined in batch assays under mesophilic Conditions (37 +/- 1 degrees C). The highest methane production rates were obtained for a mixture of 33% chemical oxygen demand (COD) vinasse and 67% COD liquid fraction of press mud and for sheer vinasse (73 and 84 N mL, CH4 g(-1) CODfed day(-1)). The maximum methane yield was found for the pure liquid fraction with a value of 369 N mL CH4 g(-1) CODfed. For most mixtures examined, antagonistic effects were found, with significant differences When mixing vinasse and the liquid fraction.