An experimental study of unsteady reacting flow in a lean premixed swirl-stabilized combustor has been performed using a high speed camera, LDV, chemiluminescence and acoustic probes. To discern the role of fluid mechanics instability, isothermal and reacting (propane burning) tests were carried out. A distinct modulation of the combustion process by the hydrodynamical instability was revealed. It was confirmed that the flow unsteadiness is attributed to the precessing left-handed spiral (helical) vortical structure. The abnormal effect of a non-monotonic evolution of precession frequency vs. swirl number, with a region of inverted (decreasing) dependence, was observed regardless of the flow conditions (isothermal/reacting, confined/unconfined). The temporal frequency of the helical mode was treated as a superposition of two terms accounting for its pure rotation and axial translation but weighted with coefficients which are azimuthal and axial wavenumbers, respectively. Using cross-correlation measurements.. the impact of swirl was confirmed to increase the helical structure's axial wavelength. This effect naturally yields the observed decreasing trend in the frequency curve due to the second contributor. (C) 2007 Elsevier Ltd. All rights reserved.