A peculiar slow relaxation of low-temperature photoconductivity in semi-insulating GaAs is analysed. Time evolutions of free electron concentration, n, and free hole concentration, p, are determined from independently measured photoconductivity and current due to the thermoelectric effect. It is shown that during low-temperature illumination the p/n ratio at first rises significantly and then decreases back towards the initial, low value. The same behaviour is obtained from a study of time evolution of photo-Hall mobility measured independently. Results are analysed and explained in terms of different trapping rates of photogenerated free electrons and holes by deep traps.