This guideline addresses the determination of the solubility in water of essentially pure substances which are stable in water and not volatile. Before determining water solubility, it is useful to have some preliminary information on the substance, like structural formula, vapour pressure, dissociation constant and hydrolysis as a function of pH. The column elution method and the flask method which cover respectively solubilities below and above 10-2 g/l are described. The test is preferably run at 20 ¬± 0,5 ºC. A simple preliminary test is allowed to determine approximately the appropriate amount of sample to be used in the final test, as well as the time necessary to achieve saturation.