A system and method for remotely determining at least one of the temperature of, and the relative concentrations of species making up, a hot fluid, based on the spectral structure of radiation emitted from the fluid. Thermal radiation over a field of view including the hot fluid is collected. At least a portion of the emission spectrum from the collected radiation is resolved. The resolved emission spectrum is resolved into spectra that are characteristic of specific emitting species and emitter temperatures. The temperature of and the relative concentrations of species making up, the hot fluid, are determined from the relative amounts of at least two resolved spectra.