Experimental studies of fuel utilisation during the early stages of engine warm-up after cold-starts are reported. The investigation has been carried out on a 1.81, 4 cylinder spark-ignition engine with port electronic fuel injection. The relationship between fuel supplied and fuel accounted for by the analysis of exhaust gas composition shows that a significant mass of fuel supplied is temporarily stored or permanently lost. An interpretation of data is made which allows time-dependent variations of these to be separately resolved and estimates of fuel quantities made. The data covers a range of cold-start conditions down to -5°C at which, on a per cylinder basis, fuel stored peaks typically at around 0.75g and a total of 1g is returned over 100 seconds of engine running. Fuel lost past the piston typically accounts for 2g over 200 to 300 seconds of running. Fuel stored is poorly mixed on return to the bulk mixture and passes into the exhaust system primarily as unburned hydrocarbons. A UEGO sensor in the exhaust gas stream (and by implication a HEGO sensor) records mixture AFR's which are lean of the overall AFR deduced by exhaust gas analysis. Correction functions based on exhaust gas hydrocarbon composition content and start temperature have been defined. During warm-up, mixture control based on HEGO feedback information will provide fuel-rich rather than stoichiometric exhaust products composition. This will adversely effect catalytic converter efficiency even if early light-off conditions are achieved.