The combustion behavior of municipal and residual petrochemical sludge and their co-combustion characteristics has been studied using differential thermal gravimetric (DTG) analysis. These experiments have focused on determining the effect of different heating rates and different sludge compositions on the co-combustion of mixed sludge feedstocks. Comparison of thermogravimetric and DTG results measured at different heating rates has enabled optimum heating rates to be identified for the combustion of mixed samples. This involved analysis of a range of combustion parameters, including devolatilization index, ignition index, burn-out characteristics and comprehensive combustion indices. These results can be used to maximize the co-combustion performance of mixed samples that contain different weight percentages of residual petrochemical sludge and municipal sludge.