Abstract: Air drawn in by a fan 4 is cooled by evaporator coils 2 and re-warmed by a condenser 3. A microcontroller MC and sensor S reads the temperature of the incoming air at regular intervals, e.g. once every minute, and controls a compressor 1 to operates in successive run periods, during which the evaporator 2 removes moisture from the air, separated by defrost periods in which the compressor is switched off so that warm incoming air melts any ice on the evaporator. The temperature at the start of a run period determines the duration of the respective run period, and the temperature at the end of a run period determines the length of the following defrost period. The length of the run period is constant at low temperatures but increases to a longer constant period at higher temperatures. The length of the defrost period is a maximum close to freezing point, but is reduced by decreasing increments as air temperature increases. At higher temperatures the dehumidifier operates continuously with no defrost.