A simple but accurate method using tritiated water for determining the efficiency of a plant for drying a halogenated, hydrocarbon oil is described. By the use of tritiated water with an activity of 50 mc/ml., it is possible to measure water contents as low as 0.04 p.p.m., while, with an activity of 5 c/ml., the sensitivity may be increased to four parts in 10¹⁰. An advantage of the method is that the samples may be exposed to the atmosphere for short periods without appreciably changing the labelled water content.