Chemiluminescent emissions from OH. CH., and C-2(.) and continuous emissions from CO2. have been measured in natural-gas-fuelled, premixed, counterflow flames operating with an equivalence ratio between 0.7 and 1.3 and strain rates between 80 and 400 s(-1), achieved by varying the exit velocity of the jet between 1 and 5 m s(-1). Cassegrain receiving optics coupled to a high-performance spectroscopic unit allowed local, temporally resolved measurements of intensity of chemiluminescence in flat flames, which were compared with measurements along a line of sight obtained from flame spectra. This study allowed independent evaluation of the effects of strain rate and equivalence ratio on intensity of chemiluminescence and the ability of intensity of chemiluminescence to indicate heat release rate. Results suggest that intensities of chemiluminescence from OH. and CH. and background intensity from CO2. are good indicators of heat release rate, whereas that from C2 is not. The study also evaluated the ability to measure equivalence ratio of the reacting mixture using intensity of chemiluminescence and found that the intensity ratio OH./CH. has a monotonic decrease with equivalence ratio for lean and stoichiometric mixtures, while remaining independent of flame strain rate. The results indicate that the intensity ratio OH./CH. can measure with uncertainties of 5% the equivalence ratio up to values of 1.1 and with uncertainties of 20% for richer mixtures due to the low sensitivity of the intensity ratio OH./CH. for rich mixtures. The intensity ratios of C-2(.)/OH. and C-2(.)/CH. have a non-monotonic dependence on equivalence ratio and a dependence on strain rate and are thus not suitable for measurements of equivalence ratio. An approach to measuring the time-dependent equivalence ratio of the reacting mixture is suggested and data processing methods and associated uncertainties are presented. The potential of the technique for local measurements in practical burners is discussed and further evaluation of the spatial resolution is required in such flames. However, suggestions have been provided on how the spatial resolution can be improved in practical flames. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.