The present work dealt with the relationships between the degradation and humification processes which the organic matter underwent during the composting of six different organic-waste mixtures. Four of them were prepared by the Rutgers forced-ventilation composting system and the other two by the mobile (turn over) pile system. The main components were: sewage sludge, sorghum bagasse and municipal solid waste. Different degradation rates were observed for the three main components (cellulose, hemicellulose and lignin) of the organic matter during composting. In the case of the first two components, the degree of degradation ranged from 70 to 85% during the whole process, depending on the starting mixture, whereas only 30-50% of the initial concentration of lignin was degraded in the mixtures prepared with municipal solid wastes (MSW) and lignocellulosic materials. Water-soluble carbohydrate and phenol degradation were studied because they have been proposed as precursors of the humification processes. In the experiment described, they had different degradation rates during composting depending on the starting mixture and the composting system used. The water-soluble carbohydrate was the most intensely degraded fraction in the piles prepared with urban refuse although no appreciable degradation was measured in the other three mixtures, whereas there was an appreciable reduction in the water-soluble phenol fraction of all six mixtures during composting, values of less than 0.1% being reached in the mature composts. Humification processes were studied by quantification of the extractable humic-like substances and the generally accepted humification indices: extractable carbon to total organic carbon (EXC/TOC), humic acid carbon to total organic carbon (HAC/ TOC), humic acid carbon to extractable carbon (HAC/EXC) and the humic acid carbon to fulvic acid carbon (HAC/FAC) and by determining the cation exchange capacity of the mixture during composting. All the indices increased during composting and followed a similar trend. The humic-like acid fraction was mainly responsible for these changes, showing that the composting involves a process of humification. The cation exchange capacity to total organic carbon ratio showed itself to be a useful humification index during composting since this index clearly increased more than the others. Correlations between some of the above humification indices and the concentrations of water-soluble and less polymerised carbohydrates and phenols indicates the possible influence of these fractions on the humification processes. Significant correlations were found between the phenols and the HAC/TOC and the HAC/EXC ratios, whereas no significant correlations were recorded with the carbohydrate fraction.