Conventional parameters (loss on ignition, total organic carbon, total nitrogen, C/N-ratio, respiration activity (RA₄), compost status (= 'Rottegrad'), NH₄-N and NO₃-N) are not correlated to humification. At best, they provide information on the biological stability (status of degradation) of composts. Humic substances which are a source of stable organic matter and nutrients are discussed as a parameter describing compost quality. Thus, in the present research project a photometric method evaluating humic acids was used to characterize the quality of 211 Austrian and foreign composts made from source-separated collected biowaste or sewage sludge. Furthermore, parameters influencing the formation of humic acids during the rotting process were investigated by implementing rotting experiments in the laboratory as well as in composting plants. The analysed composts showed humic acid contents between 2.5 and 47 %, calculated on a organic dry matter (oDM) basis. In addition to the duration of treatment the main influence on humification was the feedstock used. Stabilized sewage sludge, biowaste after intensive anaerobic pre-treatment or biowaste with low reactivity (RA₄) or uniform composition (e.g. mainly grass) showed a low formation of humic acids. For optimum humification the feedstock needed to contain components that are well balanced from scarcely to easily degradable compounds. Processing also influenced humification. Open windrow systems and reactor systems allow the same quality to be produced when operated well, but optimizing mineralization (e.g. very intensive aeration) showed negative effects. The positive condition required for humification is an unhurried (not too intense) degradation with long-lasting biological activity in which microbes have enough time to use the metabolic products of degradation for humification.