We present a heat release dynamics model which utilizes a well-stirred reactor (WSR) and one-step kinetics to describe the unsteady combustion process. The analysis incorporates the linearized mass and energy equations to describe the response of the reactor to external perturbations and is cast in the form of a first order filter. We are able to predict the phase between the mass flow rate oscillations and the resulting heat release fluctuations. as function of the operating conditions, for example, the mean equivalence ratio and mass flow rate. The model predicts a 180degrees sudden shift in phase between imposed flow oscillations and resulting heat release between the maximum reaction rate and the blow-out limit. We show that this phase change may trigger combustion instability as the equivalence ratio is lowered or the average mass flow rate is increased. A number of experimental investigations. in which the inlet nozzles were choked to minimize equivalence ratio oscillations, are used to corroborate this conclusion. Next. the heat release-mass flow relationship is coupled with the acoustic field and is applied to predict instability conditions in high swirl combustion. The predictions agree qualitatively with the corresponding experimental observations.