An experimental study has been conducted to characterize NO and soot evolution in an optically-accessible D.I. diesel engine with a square combustion chamber. Two-dimensional laser-induced fluorescence was used to characterize NO evolution. Soot evolution was characterized by two-dimensional laser-induced incandescence (LII) and Mie scattering techniques as well as direct photography of the flame luminosity. The engine operating parameters were set to provide optimum conditions for NO imaging. Attenuation of the UV beam proved to be the major obstacle in obtaining NO images. Therefore, oxygen was added to the intake air charge in order to reduce the optical density of the combustion medium. The NO images showed that the NO formation started almost immediately after ignition and ceased no later than 40 degrees ATDC. No soot images could be obtained by the laser-induced incandescence or Mie scattering methods before 20 degrees ATDC since the soot concentration was very low.