Quantitative images of vapor-phase fuel concentrations were obtained in an evaporating and combusting diesel jet using planar laser Rayleigh scattering. The diagnostic has been calibrated, evaluated, and successfully applied to an optically accessible direct-injection diesel engine for fired and nonfired operating conditions. The measurements were obtained in the leading portion of the diesel jet (the zone beyond 27 mm from the injector nozzle), where the fuel is entirely evaporated, and which corresponds to the main combustion zone in this engine. The technique was shown to be effective for quantitative imaging of the fuel-vapor concentration before ignition, with high spatial and temporal resolution. Additionally, images of the fuel-vapor concentration were further reduced to imagers of the equivalence ratio using an adiabatic mixing assumption to model the local temperature of the evaporating diesel jet. This procedure also yielded temperature distribution images. The results show that, at 4.5 degrees crank angle (0.63 ms) after the start of injection, which corresponds to the time just before the first indicated heat release, the fuel and air are relatively well mixed in the leading portion of the diesel jet. At this crank angle, the equivalence ratio in the majority of the jet ranges from 2 to 4. The edges of the jet are well defined, with the signal level rising sharply from the background level up to levels corresponding to equivalence ratios in the jet. The temperature of the richest mixture regions in the jet is as low as 700 K, with the ambient air temperature at 1000 K. Finally, comparisons of Rayleigh images of the reacting and nonreacting jet show that the initial breakdown of the fuel, indicated by a significant decrease in the Rayleigh signal intensity, occurs throughout the cross section of the leading portion of the diesel jet.