Photometric observations of trace additives have been used to measure radical concentrations in some fuel-lean hydrogen-nitrogen-oxygen flames. The NaCl and CuH methods of measuring [H] have proved to be inapplicable to these flames, but the LiOH method for [H], the NaOH method for [OH] and the IO (or BrO) method for [O] have been used successfully. It is believed that the LiOH method is not seriously affected by LiO2 formation. Evidence in support of a value of 104 kcal/mole for the dissociation energy of LiOH has been obtained. Radical recombination or chemiluminescent excitation processes involving the recombination of oxygen atoms to form O2 are considered.