The current work seeks to determine the feasibility of using radicals to improve the combustion of propulsion systems fueled by hydrogen. An ArF excimer laser (193 nm) with focused optics was used to photodissociate molecular oxygen in hydrogen/oxygen mixtures, producing ground-state atomic oxygen, O(3P). In these experiments, the laser penetrated more than 20 cm into the mixture, there producing a maximum atomic oxygen concentration which leads to pure photochemical ignition. The minimum laser pulse energy required for ignition has been determined over a range of equivalence ratios (0.2 to 2.0) and initial temperatures (400 to 520°C) at a baseline pressure of 0.5 atm. Based on these values, the atomic oxygen concentration produced in the focal volume has been calculated. For these conditions, both the minimum laser energy and atomic oxygen concentration decrease strikingly as the initial mixture temperature increases. Results from a simplified ignition model agree with the experimental data. Both show only a weak dependence of the atomic oxygen concentration required for ignition on the equivalence ratio of the mixture over the ranges studied