Ensemble averaged heat release rates and pressures during pulsating combustion have been measured to better understand the operating characteristics for a twin valveless pulse combustor with two aerodynamic valves. An OH chemiluminescence intensity detection system is applied to determine the periodic variations in heat release. Also, the combusting flows have been visualized using high-speed schlieren photography. Based on the well-known Rayleigh criterion, the varying equivalence ratio and mass flux effects on combustion stability are discussed. Results show that the phase relationship between the heat release rate and the pressure significantly affects the pulse combustion stability and operating frequency. Of particular interest is that the pulsating frequency decreases with increasing equivalence ratio. In addition, the phase difference does not depend on the mass flux, but on the equivalence ratio. This fact indicates that the fuel-air mixing rate is a determinant factor in the phase relationship. The importance of fluid dynamic mixing in pulsating combustion is successfully illustrated by the schlieren observations.