Abstract: Modified gelatin is widely used as coagulating agent in photographic emulsion-making, but the coagulation mechanism is still obscure. The isoelectric point distribution is one of the most important factors affecting the coagulating property of modified gelatin. In the present work the new technique of isoelectric focusing with some modifications is used in the determination of the isoelectric points of parent and modified gelatins. The experimental data can be used to account for the reason why modified gelatin and the photographic emulsion prepared with it can be coagulated only within a definite range of pH.