Abstract1. An outline of the leaching rate method of studying antifouling compositions is given, together with a consideration of the sources of error in such work.2. Details are given of the method used in the present investigation, in which a large number of compositions have been tested.3. By the annlysis of variance technique the coefficient of variation for copper leaching rates is calculated from the data of a number of series of compositions.Over an exposure period of 215 days for a representative series of cuprous oxidc compositions the C.V. is 13.6°. It is shown that the value increases with the cxposurc pcriod and it is suggested that this is due to an increase in the errors arising from the presence of fouling on the test slides. This is substantiated by the higher value obtained from a series with inadequate anti‐fouling properties, and by the lower value from a series always adequately antifouling.4. The above values are compared with that obtained from a series of compositions based on phenarsazine chloride. An increase in the variability with the latter poison is ascribed to the more difficult and protracted analytical technique.5. Analyses are given indicating the variability arising from laboratory work. These arc shown to be comparatively small compared with the total variability. It is therefore suggested that further refinements in the leaching rate technique are not necessary unless the errors arising from the composition and painting as well as from the ex osure conditions are brought under some more adequate control.