The effect of natural zeolite on methane production from secondary paper and pulp sludge and its modeling are evaluated. Five tests with zeolite concentrations of 0, 0.2, 0.5, 1 and 20 g/L were evaluated at 30 degrees C. The modified Gompertz equation, the logistic function and the transfer function were evaluated. The results show that doses between 0.2 and 1 g/L of zeolite produce a statistically significant increase of accumulated methane, giving values greater than 183 mL for CH4/g of VSadded. On the contrary, the 20 g/L dose of zeolite reduced the methane production. The modified Gompertz and the logistic function models concur with the experimental data, the former having the best fit. (C) 2014 Elsevier B.V. All rights reserved.