To gain a better understanding of mixture requirements during starting, a diode laser based spectroscopic technique was developed to simultaneously measure the cycle-by-cycle fuel vapor-air equivalence ratio and residual gas CO2 concentration inside the cylinder of an operating engine. Cranking to startup conditions were simulated in a single-cylinder CFR engine installed in a cold test facility. In separate tests using propane, isopentane, and gasoline as fuel it was found that combustion began in the first cycle in which the fuel vapor-air equivalence ratio exceeded the lean flammability limit of the fuel. In the range of temperatures 22°C to -12°C, richer mixtures were required to start the engine and keep it firing consistently at lower temperatures. Intake charge dilution caused by the residual burned gas left over from the combustion in a previous cycle was found to contribute to misfires in some of the succeeding cycles. Cycle-by-cycle variations occurred in mixture strength as well as residual gas CO2 concentration in all the tests.