The development of non-conventional combustion technology with ultra-low emissions and the safe operation of combustion systems require a thorough understanding of the mechanisms of combustion instabilities. The objective of the present work is to investigate the role of unmixedness and chemical kinetics in driving combustion instabilities. The reaction-rate responses of different species to inlet flow variations have been studied using a perfectly stirred reactor model. Transient simulations of combustion of methane and propane with air, using both global single-step and detailed chemical kinetic mechanisms, have been conducted with imposed oscillations on inflow mass flow rate, temperature, and mixture equivalence ratio. The detailed mechanisms predicted fuel reaction-rate oscillations with amplitudes proportional to the imposed oscillations. However, increased amplitudes of the reaction rates of CO2 and OH were observed when the combustion became leaner, while the reaction-rate amplitudes of CO and H-2 decreased. The single-step mechanisms predicted to some degree a similar reaction-rate behavior as the detailed mechanisms. However, near stoichiometric conditions, the fuel reaction rate of propane showed little influence by the imposed oscillations. When the mean equivalence ratio was lowered below a certain value, the fuel reaction-rate oscillations grew stronger and became larger than those seen with the detailed mechanism. This shows that simple mechanisms can by themselves introduce instabilities not seen with detailed mechanisms.