Abstract: A process for determining the pH value in the interior of a cell is described. This is accomplished by measurement of the emission of a fluorescent substance which is absorbed by the cell through incubation in a solution. The solution contains a compound of the said fluorescent substance; the fluorescent substance is separated from this compound in the cell by an enzyme present in the cell. The fluorescent substance selected is one such that the frequency of the emission maximum is dependent on the pH value. The compound of this fluorescent substance which is selected is one which is membrane penetrating. Measurement of the fluorescence is based on the principle of measurement of the frequency at which the emission maximum occurs. A fluorescent substance suitable for this purpose is 2,3-dicyano-1,4-hydroquinone. A suitable membrane penetrating compound is 1,4-diacetoxy-2,3-dicyanobenzene or 1,4-dibutyryloxy-2,3-dicyanobenzene or 1,4-di(-tert-butyloxycarbonyl-1-alanyloxy)-2,3-dicyanobenzene.