The application of planar laser-induced Rayleigh scattering for quantitative 2-D measurements of vapor-phase fuel concentration in the main combustion zone of a direct-injection Diesel engine has been explored, developed and demonstrated. All studies were conducted in an optically accessible direct-injection Diesel engine of the “heavy-duty” size class at 1200 rpm and motored TDC conditions which were typical of the production version of this engine.First, this study verifies that beyond 27 mm from the injector all the fuel is vapor phase. This was done by investigating the Diesel jet under high magnification using 2-D elastic scatter imaging and subsequently evaluating the signal intensities from the droplets and other interfering particles (Mie scattering) and the vapor (Rayleigh scattering). Then, in this vapor-phase region, which corresponds to the main combustion zone in this engine, planar laser-induced Rayleigh scattering was applied to obtain quantitative 2-D fuel-vapor concentration images of the fuel-air mixture from 4.0° to 5.5° after start of injection (ASI). At 4.5° ASI, which corresponds to the time of first indicated heat release, the fuel and air were relatively well mixed throughout the leading portion of the Diesel jet. The equivalence ratio in the majority of the jet ranged from 3 to 5.5. The edge of the jet at the front and along the sides was well defined with the signal level rising sharply from the background air level up to levels corresponding to equivalence ratios of 4 to 5. By 5.0° ASI, the fuel vapor concentration in the front of the jet has decreased sharply to near stoichiometric values indicating rapid mixing of the fuel with the air. Then at 5.5° ASI extremely low Rayleigh signal intensities in the images suggest regions of early premixed combustion with no evidence of soot formation at these crank angles.