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This paper addresses issues associated with the accurate determination of absolute cylinder pressure in internal combustion engines. Pressure referencing errors are shown to produce large errors in derived parameters such as polytropic index, mass fraction burned and charge temperature. Two alternative pressure correction methods, namely inlet manifold pressure and polytropic index referencing are investigated in detail. Sources of errors and algorithm improvements are investigated and discussed. Comparison between the two pressure referencing techniques is made using measured cylinder pressure data obtained from a gasoline engine operating over a wide range of speeds and loads. The analysis shows that both of the methods should be capable of referencing typical experimental pressure data to within +-100 mbar. The work has demonstrated that accurate absolute pressure referencing can only be achieved if common pressure measurement errors are minimised.