Three simplified combustion systems have been investigated for acoustical oscillations. The systems are: (a) a mixing chamber and multiple-port burner mounted in a combustion tube open at both ends; (b) a similar mixing chamber and multiple-port burner which completely blocked the secondary-air inlet of the combustion tube; and (c) a system similar to (b), but utilizing separate fuel and primary-air inlets, giving a diffusion flame rather than a premixed-gas flame. Oscillations were found to occur only for certain ranges of the variables. In the first system, the pitch differed significantly in adjacent ranges of oscillation. In the two latter systems, the frequency of the oscillations remained fairly uniform and close to the fundamental of the combustion tube, although breaks occurred in the data as the configurations of the inlet supply systems were varied. These results are explained on the basis of (a) the time of feedback of pressure pulses which ultimately cause changes in the heat-release rate, and (b) the location of the flame relative to a section of maximum pressure amplitude in the combustion tube. In further substantiation of the implications of the test results, the Appendix presents a thermodynamic treatment of the phase requirement between rate of heat release and pressure.