The ascorbic acid content was determined in 25 different diet supplements commercially available to the consumer in two pharmaceutical forms (pills and ampoules). These products are widely consumed by several population groups (elderly people, sportsmen, adolescents, children, etc.). High-performance liquid chromatography was used as the analytical technique. The proposed method has been validated with good linearity, reproducibility, recovery and accuracy, and can be used in routine analyses and in quality control. The ascorbic acid content in pills ranged from 15.62 to 50.16 mg/g, and in that ampoules from 2.12 to 8.83 mg/ml. Depending on the dosage rates, these levels would represent approximately 20-50% of the daily dietary intake recommended by the National Research Council. Possible losses in the manufacturing process and stability during storage for 30 days at 40 +/- 2 degrees C without light, were tested. In relation to the ascorbic acid concentrations stated on the labels, a loss of 12.0-21.9% in pills and of 11.7-18.0% in ampoules was detected. In relation to the stability conditions, the losses are of 1.8-24.8% in pills and of 10.4-19.3% in ampoules. The pasteurisation and sterilisation processes produced a mean loss of ascorbic acid in ampoules of 2.1 and 1.4%, respectively. A statistically significant direct correlation was observed between ascorbic acid loss and content in proteins, humidity, ash, and fructose. The influence of the pharmaceutical form was also tested. Data revealed that the control of losses during the manufacturing and commercialisation process of these products is necessary to ensure the intake of vitamin C from these products by the consumer.