In the paper, the effect of speech interference on the performance of long-loop phase-locked VHF SSB radio systems is evaluated, It is shown that unless the system's reference tone is adequately protected by suitable filtering in the transmitter's audio processing, the problem of ‘speech puffing’ may occur with severe degradationin intelligibility. Furthermore, residual FM modulation of the VCO's control line (the first local oscillator) through speech interference degrades the oscillator's output spectrum with possible reduction in adjacent-channel rejection. After analysing the effect of interfering sinusoidal signals on in-lock-loop per formance, filter design criteria are established for pilot-carrier and in-band-tone SSB systems, and the theoretically predicted results are verified experimentally. Finally, alternate phase-locked receiver configurations are suggested in an attempt to overcome first-oscillator spectrum degradation in long-loop receivers.