The introductory part of the paper shows the importance of the problem of sewage sludge utilization, which is still not fully solved in some countries. Thermal methods of sludge utilization were shown as satisfactory final solution of the problem. The specific Polish situation, with many small sized coal fired boilers leads to co-combustion as possible solution and to necessity of investigation of sludge parameters during its storage. The results of investigation of parameters of sludge originated from large communal sewage treatment plants during simulated open air storage for 8 weeks period are presented. The measured parameters were: combustible parts, volatile parts, Lower Heating Value, carbon contents. The correlations between measured parameters and storage time were determined.