Tien's model for unsteady gas phase analysis has been adopted for the oscillatory burning of homogeneous solid propellants. The gaseous flame is assumed to be one dimensional, premixed and laminar. Three overall chemical processes simulating the fizz zone and the final flame zone (including dark and luminous zones) are considered to take place in the gas phase. The propellant is assume to decompose according to the Arrenhius law with no condensed phase reaction. The strong gas resonance obtained by T'ien was recovered. The three chemical reaction analysis resulted in an extended flame zone and produced an acoustic admittance which differed from that obtained from Tien's one-step chemical reaction at both low and high frequencies. There appear two peaks: one. at a lower frequency, corresponds to a resonance occurring in the condensed phase to amplify the pressure oscillation; the other, near the gas-phase characteristic frequency, is in the direction to damp the pressure oscillation.