In this study, after nine cylindrical bales containing a mix of different waste materials were constructed, they were stored in the open air and temporal variations of CO2, CH4, O-2, and N-2 were monitored over 10 months. In each bale, different waste fractions were considered in order to represent different moisture contents. The results showed that CO2 increased within very few days to approximately 80% and stabilized later in the range of between 10% and 35% in a month. The O-2 levels dropped from approximately 15% to significantly less than 1%. There was no significant anaerobic decomposition since CH4 did not exceed 5% during the whole test period. N-2 exhibited an opposite pattern with CO2. In addition, relationships between waste species in the bales and gas formations were determined by a bivariate correlation analysis. An empirical prediction model for the maximum CO2 production was also developed. (C) 2012 Elsevier Ltd. All rights reserved.