The vapor diffusion and the combustible mixture formation around evaporating fuel droplets are studied. The formulas derived for the droplet temperature and vapor concentration profiles take into consideration the unsteady period before the droplet reaches its equilibrium temperature. In this model the droplet is assumed to be suddenly brought into contact with a high temperature oxidizing atmosphere.The ignition delay is considered equal to the period of time from the start of heating up to the time of formation of a stoichiometric mixture at the ignition location around the droplet.Computations are made for the temperature history, concentration history, and the ignition delay for iso-octane droplets evaporating in air. The droplet sizes considered are between 1120 and 1520 μ. The air pressure is 14.7 psia, and the air temperatures are 1095-1390 F. The comparison between the results of the present model and previous experimental results showed favorable agreement.