The fuel/air ratio (FAR) in a methane-air Hencken flame at pressures of 1-5 bar is measured using the chemiluminescence-based method. Emission spectra are used to investigate the effects of pressure on the OH* (308 nm), CH* (430 nm), and C2* (500 nm) emissions and the effect on equivalence ratio determination from the ratios of these emission peaks. Both OH*/CH* and C2*/CH* ratios are linear to FAR at atmospheric pressure. At elevated pressures, C2*/CH* remains roughly linear to FAR, while OH*/CH* becomes highly nonlinear. There are significant spectral contributions from continuum radiation at higher pressures, likely due to increasing soot production. Therefore, while it is a truly passive and nonintrusive diagnostic method, the use of chemiluminescence for FAR determination at high pressures could be limited. Possible improvements to the measurement setup and future studies are discussed.