The chemiluminescent emission measurements of three active radicals (CH*, C-2* and OH*) of a diffusion flame jet in an acoustically excited cylindrical tube have been investigated. By using optical integration and a bifurcated optical fibre bundle, the chemiluminescent emissions of two radicals could be measured globally and simultaneously. As a result, the cross correlation and comparison of the two measured active species could be carried out. A computer controlled traverse system has been used to adjust the relative position between the fuel nozzle and the cylindrical tube so that the flame properties at the various positions (phase) of the forced standing wave could be investigated. Digital imaging of the CH* and C-2* provides extra spatial information. It has been observed that flame properties respond very differently to the phase of a standing wave. The most dramatic flame change was observed to occur in the rarefaction zone, in which the flame jet length is the shortest and blue in colour either partly or completely. The spectrum analyses and cross correlation of the simultaneously measured CH* and OH* signals show that the two species do not correlate strongly under many conditions. Also, there exists a phase difference between the two measured species even in the case that the two signals have the same frequencies. Care has to be taken in applying either of the species as the heat release indicator, especially in analysing the unsteady combustion phenomena. (c) 2005 Elsevier Ltd. All rights reserved.