SYNOPSISThe Rassweiler and Withrow method for determining mass fraction burnt from S.I. engine cylinder pressure data is widely used, but implementation details vary. The best form of implementation, and the effect of uncertainties in the assumptions made, have been investigated. Attention has been given to determining the end of combustion, the effect of signal noise on the pressure data, the effect of assumptions regarding the value and variation of apparent compression and expansion polytropic indices and, particularly, how these should be determined. The treatment of late burning cycles is also discussed. Versatile software has been developed for idle to full load studies with good tolerance to signal noise.