I two previous papers (1, 2), experimental data relating to the problems of combustion instability in 3-in. diam rocket motors were presented. Two types of axial oscillations were identified: (a) A low frequency type of instability (220-360 cps) which is distinguished by abrupt and severe variations in radiation intensity, (b) A high frequency type of instability (600-1500 cps) which corresponds to the propagation of waves longitudinally along the motor. In extreme cases these pressure pulses were transformed into shock waves. In the present paper, additional experimental evidence will be presented relating to these phenomena. All motors were 3-in. ID. Two different injector heads were used: a conventional impinging and a modified impinging (like-on-like type) injector. As described in (1), each injector head uses an injector plate consisting of a series of concentric rings containing drilled injector holes; alternate rings being for fuel and oxidizer. Pairs of liquid jets impinge approximately one inch downstream of the injector plate in the case of the conventional impinging injector, while for the modified impinging head, the impingement point is in the plane of the plate. Table 1 lists details of the injectors. The reactants used in all tests were ethyl alcohol and liquid oxygen.