The applicability of Folin reagent developed by Tsap [12] for the determination of pure ascorbic acid solutions was checked for more complex solutions as vitamin preparations, fruits and vegetables. The AA can be determined in vitamin preparations without any modification. However it must be modified in the case of plant extracts (fruits and vegetables) as follows: one sample is determined after strong alkalization of the sample, another one without alkalization. The result of AA concentration can be achieved by subtraction of these two determinations. Good results were obtained for lettuce, parsley (tops), Brussels sprouts, lemon, orange, grapefruit, chives, kiwifruit and spinach.