Changes in several biochemical parameters, exchangeable [Formula: see text] and [Formula: see text] were evaluated during the composting of sewage sludge and city refuse. Except for casein-protease activity, all enzyme activities were higher in sewage sludge than in city refuse, at all times, denoting larger microbial populations; this was also confirmed by a high initial ATP level. In addition to differences in microbial biomass, the lower enzyme activity in city refuse might also depend on inhibition by a high content of heavy metals. Absolute urease and phosphatase activities decreased during composting, showing a minimum at 91 and 65 d, respectively; thereafter they increased slightly. ATP also decreased markedly during the first month, then it stabilized to a low level. The correlation found between urease-phosphatase-ATP and the lack of correlation between proteases, and between both proteases and ATP-urease-phosphatase, confirmed the different synthesis and behaviour of these enzymes. Proteases might partially exist outside living cells, thus playing an important role in composting when microbial activity is low. High levels of ATP and enzyme activity at the beginning of the composting, their sharp decrease and the increase in casein-protease followed by a decrease, characterized the "mineralization" phase, while low levels in ATP and casein-protease and a re-activation of the other hydrolases, characterized the "maturation" phase of the composting. The ammonium content decreased during composting while nitrate increased, indicating that the process evolved exhaustively and under fully aerobic conditions. Key words: ATP, city refuse, enzyme activity, hydrolases, sewage sludge