This paper presents the results of a study of the potential causes of frequently observed combustion instabilities in low NOx gas turbines (LNGT) that burn gaseous fuels in a premixed mode. The study was motivated by indications that such systems are highly sensitive to equivalence ratio perturbations. An unsteady well-stirred reactor model was developed and used to determine the magnitude of the reaction rate and heat release oscillations produced by periodic flow rate, temperature or equivalence ratio perturbations in the combustor's inlet flow at different mean equivalence ratios. This study shows that the magnitudes of the reaction rate and heat release oscillations produced by these perturbations remains practically unchanged, decreases, and significantly (i.e., by a factor of 5-100) increases, respectively, as the equivalence ratio decreases. These results strongly suggest that equivalence ratio perturbations, which are an indication of reactants unmixedness, playa key role in the driving of combustion instabilities in LNGT operating under lean conditions.