The stability of vitamin C, α-tocopherol, retinol and β-carotene in mash and pelleted mixed feed was studied during 12 weeks of guaranteed time of storage. Crystalline and protected ascorbic acid (Monophosphate, Lutavit® C Monophosphate 35) were the sources of vitamin C. A dietary supplement of 200 mg/kg AA was used. The analysis of vitamin C concentration was carried out in interval 14-day, with other vitamins at the beginning and at 4 and 12 weeks of storage. The interaction between the source of vitamin C and the pelleting, between the source of vitamin C and the storage time and the combined effect of the pelleting and the storage time was ascertained in the vitamin C content in the feed mixture. Monophosphate was highly stable in the mixture until the 10th week of storage and seems to be an appropriate source of vitamin C in animal feed. Crystalline AA had very low stability. Pelleting of the diet had a different effect on the stability of the monitored vitamins.