Simulation studies indicated that the start and end of combustion in an internal combustion engine could be determined from the points of minimum and maximum entropy in the cycle. This method was further used to predict the beginning and end of the combustion process from experimentally obtained pressure crank angle data from a natural gas operated, single cylinder, spark ignition engine. The end of combustion always matched well with the point of maximum entropy. The start of combustion could be determined easily from the rate of change of entropy, which showed a sharp change at ignition. Results closely agreed with those obtained from a heat release analysis.