Fluorescent dopants which allow the two-dimensional, real-time, non-intrusive visualization of vapor/liquid concentrations and droplet temperatures in hydrocarbon fuel sprays have been demonstrated. These systems, which are based on the photophysics/chemistry of organic exciplexes (excited state complexes), when coupled with planar laser excitation and electronic array detectors, should make possible the development of engineering test rigs for the evaluation of spray nozzles. Two organic compounds are added to the hydrocarbon fuel, a fluorescent monomer M and an appropriately chosen ground state reaction partner G. Electronically excited M may react with G to form the exciplex E* according to the following reaction: M* + G <==> E* In the vapor/liquid visualization studies, the equilibrium is adjusted so that the M* emission is a marker for the fuel vapor and the E* emission, which is red-shifted from the M* emission, is a marker for the liquid. In the temperature visualization studies, the temperature dependence of the M*/E* ratio is used to determine the droplet temperatures. Preliminary studies on fuel sprays of hexadecane, showing the liquid and vapor phases separately, will be presented. Exciplex-based thermometer systems applicable to temperatures as high as 400°C will be discussed.