Six composts from different sources (wheat and barley straw; coniferous bark; mixed hop rape and bark; two from household garbage; mixed paper dust and sewage sludge) were characterized by chemical methods, including various forms of N and organic matter fractionation, and by microbiological methods. The dehydrogenase activity, respiration rate, and arginine ammonification were investigated, which represent different aspects of C and N metabolism. Only a few significant correlations were found between different maturity indexes. Dehydrogenase activity was in agreement with the widely accepted humic acid C to fulvic acid C ratio and might therefore be acceptable as a maturity index. Arginine ammonification provided valuable information on the N status in composts; negative values indicated that considerable amounts of easily degradable organic compounds with wide C: N ratios were still present, which would lead to microbial immobilization of soil N after the application of these composts. A combination of dehydrogenase activity and arginine ammonification data led to an unambiguous classification of all composts, and is therefore recommended for further consideration; in contrast, chemical data were contradictory and markedly dependent on the original substrates. The respiration rate, which was closely correlated with the percentage of organic C, did not contribute to the assessment of compost maturity.