Quasi-synchronous operation of a number of base transmitters is often used to provide extensive area coverage in land mobile radio systems. An analysis is made of the responses of an AM receiver operating in such a system. The effects of automatic level control and the muting action are shown to nullify any predictions made of system performance based on harmonic distortion of sinusoidal modulating tones. A limited analysis of the types of distortion likely to arise from the mismatch of modulation between transmitters is made in order to give a guide to the parameters to be explored in subjective trials. The results of, first laboratory-based and subsequently field-based, trials are presented and from them recommendations are made for the allowable limits which should be used for the design and maintenance of a quasi-synchronous AM system.