Chemiluminescence sensing is shown to be a useful approach for monitoring local, reaction zone equivalence ratios. For natural gas combustion, the ratio of CH∗ to OH∗ chemiluminescence is shown to be a useful indicator of equivalence ratio (ϕ). The ratio of peak intensities (CH∗/OH∗) is shown to monotonically increase with ϕ for the operating regime studied (0.6 0.8) mixtures. In addition, the sensitivity of CH∗/OH∗ to changes in ϕ decreases with pressure. A swirl-stabilized, liquid-fueled (n-heptane) combustor was also investigated. Volume-integrated measurements in this nonpremixed combustor show that C2∗/OH∗ is a more sensitive measure of ϕ for complex fuels, compared to CH∗/OH∗, with measurable C2∗ signals even in lean mixtures (unlike the natural gas results). In addition, the chemiluminescence sensing technique successfully tracts the variation of fuel–air ratio with axial location in this nonpremixed combustor.