Heat release analysis from measured pressure - crankangle data in D.I. diesel engines is subject to many different sources of error. Two specific problem areas are investigated and discussed in the context of trends noted from a large number of experimental results taken from a Ricardo E16 1.6 litre single cylinder engine. These arise from the fact that: * (i) pressure is not always uniform across the cylinder at a given instant, and the values measured by one transducer may not be representative, and * (ii) the in-cylinder heat transfer correlations commonly used do not appear to adequately reflect trends in radiative heat transfer. This causes serious discrepancies between the calculated gross heat release and the calorific value of the fuel injected.