This article deals with the application of OH planar laser-induced fluorescence (PLIF) to the study of high-pressure cryogenic flames (p = 6.3 MPa). It is shown that this optical diagnostic provides good-quality data and can be used to examine the flame structure in the injector near-field. High-pressure conditions require a careful choice of excitation wavelength based on a detailed analysis of the absorption coefficient dependence on pressure and temperature. It is particularly important to consider line-broadening and central-frequency-shifting effects induced by high-pressure operation. PLIF is used to examine jet flames formed by a single coaxial injector fed by liquid oxygen and gaseous hydrogen (LOx/GH(2)) in subcritical and transcritical regimes. In the latter case, the liquid oxygen temperature is below its critical value while its pressure is above critical (T-LOx = 85 K < TC (02) = 150 K, PLO, = 6.3 MPa > Pc(02) = 5.04 MPa). Such transcritical conditions prevail in many high-performance devices such as liquid-propellant rocket engines. A detailed understanding of this type of combustion is necessary to the development of improved and more reliable propulsion systems. PLIF provides images that may be considered to represent instantaneous distributions of OH radicals and may be used to infer the structure and position of the flame. An optical multichannel analyzer is also used in the experiments to ensure that the detected signals have the expected spectral features. Fluorescence spectra are also compared with emission spectra synthesized with Lifbase in terms of line positions and broadening. Mean flame positions deduced from PLIF images are shown to nearly match those deduced from the Abel transformed average OH* light-emission distributions. The difference is related to a biased estimate of the flame position resulting from the Abel transformed emission data. PLIF is also used to examine the flame edge. It is known from a previous analysis of the flame-holding mechanism that the low-speed wake established just behind the oxygen injector lip and generated by the two propellants must be thicker than the flame-edge thickness to ensure stable anchoring. PLIF images indicate that the flame thickness is on the order of the wake transverse size. When the thickness exceeds this transverse dimension it is found that the flame becomes sensitive to the high-speed hydrogen stream. Finally, the instantaneous PLIF images of OH included in this article may serve as a guide to current efforts directed at the large-eddy simulation of transcritical combustion. (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.