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 < / < 1.2). A "universal" relationship between CH∗/OH∗ and φ was demonstrated, for the first time, in various natural gas combustor configurations ranging from unconfined jet flames to swirl- and dump-stabilized combustors. This relationship was also not sensitive to moderate changes in flowrate (velocity), air preheating, and fuel composition (methane vs. natural gas). The universal dependence of CH∗/OH∗ on φ is obtained by correcting the peak chemiluminescence intensities for broadband background radiation associated with other gases (e.g., CO2∗) and hot surfaces. Pressure does impact CH∗/OH∗; the results indicate an increase in the ratio with pressure (for fixed φ). There may be a nonmonotonic behavior below 3 bar for near stoichiometric (φ > 0.8) mixtures. In addition, the sensitivity of CH∗/OH∗ to changes in φ decreases with pressure. A swirl-stabilized, liquidfueled (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. © 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.