Photochemically augmented combustion has been identified as a potential technique for extending combustion associated limits. The objective of this research has been to investigate initiation of combustion by irradiation of unsensitized fueloxidizer mixtures with ultraviolet light. This light, absorbed initially by oxygen molecules, results in their photodissociation to oxygen atoms, which produce other reactive radicals via chain-branching. The presence of these radicals and subsequent thermalization results in ignition. In this study, a Lumonics excimer laser (TE 861) is used to investigate the effect of wavelength on the photochemical ignition of hydrogenair and hydrogenoxygen mixtures at several equivalence ratios and pressures. Two wavelengths are used; 157 nm emitted by fluorine and 193 nm emitted by argon fluoride. Photoignitions were achieved with the fluorine laser and not with the argon fluoride laser, even though the latter has 10-fold greater fluence. This result demonstrates the important role of the selectivity of the absorption coefficient of molecular oxygen, which sharply decreases (four orders of magnitude) as the wavelength increases from 157 nm to 193 nm. Experiments in which minimum pressures for ignition of H 2 O 2 and H 2 air mixtures were determined at various equivalence ratios suggest fundamental differences between the performance of photochemical and conventional spark ignited systems. Overall minima for photochemical initiation occurred at equivalence ratios to the fuel-lean side of stoichiometric. This fuel-lean shift appears typical of photochemical initiation, with the explanation that increased molecular oxygen yields higher atomic oxygen concentration upon irradiation. A complementary analytical effort was undertaken to eludicate the fundamental interaction of photon absorption, subsequent dissociation, chemical kinetics, and heat loss to surroundings. These processes are described by energy and species conservation equations, including some 90 chemical reactions and 6 photodissociation reactions, the latter allowing for the temporal and spectral dependence of the incident light and absorber cross sections. Good quantitative agreement with the experimental data of the laser ignition experiments attests to the adequacy of the model for investigation of photochemical initiation and supports the postulated mechanism.