Some design aspects of the Ariel HI v.h.f. communications system are described with particular emphasis on the measures taken to enhance reliability. Throughout the design and development of the satellite common-user electronics equipment, component and circuit redundancy techniques have been employed wherever practicable in order to achieve the maximum degree of reliability. These redundancy techniques can be applied to low-frequency circuits without serious degradation of performance; at v.h.f., however, the techniques create intolerable difficulties.The command receiver, transmitter, and other units comprising the v.h.f. system were designed for maximum reliability, using redundancy techniques wherever possible. In addition to these measures, a stand-by transmitter was incorporated. The implications of this major modificationin terms of its improvement to overall reliability, and its effect upon the command system is discussed.As a common aerial system was used for the command receiver, and the telemetry transmitter, sufficient isolation of the transmitter was required to maintain the command receiver sensitivity. A solution adopted for this purpose is mentioned and this is followed by a general description of the command system, telemetry link and constructional techniques. Operational results since launch are included.