In practical implementation of sewage sludge composting process a very important technology factor is a share of materials with high C content (sawdust, straw, bark, etc.). The influence of these additives does not consist only on gaining of suitable, porous heap structure but also has a direct interspersion on the C:N level in composting heaps. Many authors show that the best C:N proportion in composting process amounts between 20-30, whereas the lower one (below 15) can lead to excessive ammonia losses as a result of ammonia emission. Meanwhile the national economic conditions, in particular high price of straw and wood wastes, causes that on composting plants there are some trials of reducing the participation of carbonic additives in order to avoid excessive costs. In consequence it can lead to a very strong increase of ammonia emission. The paper presents the research on composting process run and ammonia emission from sewage sludge composted in mixtures with straw and sawdust in a share of 75, 60, 45 and 30% of dry mass. It has been stated that along with a decrease of sewage sludge participation the C:N relation extends and N losses decrease as a result of ammonia emission.