Experimental data of supersaturation measurements in the seeded water solutions of L(+)-ascorbic acid during its batch mass crystallization with adjustable cooling system providing a constant gradient of temperature are presented. The tests were performed in a laboratory-scale DT MSMPR crystallizer with internal circulation of suspension created by a propeller agitator. Experimental data suggest that the complex supersaturation profiles within investigated solutions (with maximum) strongly depend on the initial concentrations of acid (its saturation temperatures) and on the cooling rates.