Abstract The spiny rat mite has been found to have a mean half-life of water exchange with vapour in the surrounding air of 18·6 hr at 25°C when the activity of the water vapour of the air a v = 0·925. Tritiated water was used in making this determination. It was found that the rate constant for transpiration ( k T ) could be determined by measuring changes in the tritium content ( T ∗ ) of the mite with time and that the rate constant for sorption ( k S ) could be similarly calculated from determinations of the specific radioactivity ( S). The minimum rate constant ( k P ) for the pump responsible for the concentration of water from unsaturated air by the spiny rat mite can be calculated from k T , a w of the haemolymph, and the critical equilibrium activity. These studies were done under equilibrium conditions so that no significant changes in mass occurred. The mean values of the rate constants were: k T=k S0·0373 hr −1 and k P=0·0036 hr −1 .