Communal sewage sludge was composted in an open-air compost plant with the addition of sawdust, wood chips and straw. Sewage sludge was composted in the compost heaps with: sawdust, with sawdust and wood chips or with sawdust and straw. The entire experimental material was mixed with the tractor aerator which was also used to fragmented the material and form compost heaps. During the experiment, samples for analyses were collected on the day of experiment establishment as well as after 21, 36, 49 and 90 days with the aim to determine, among others: contents of dry matter, organic matter and organic carbon. It is evident from the obtained data that changes in the content of the examined substances were influenced, primarily, by the time of composting and, to a lesser, albeit significant extent, by the cooperation of time and compost heat composition. In the case of organic matter and organic carbon, the compost heap composition failed to exert a significant impact on their content and changes during composting.