The stability of vitamin C in mash and pelleted mixed feed was studied. Feed without the addition of ascorbic acid (AA) and mixed feed supplemented with 200 mg/kg AA in the form of crystalline or protected AA (Lutavit® C Monophosphate 35) were analysed. 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. In contrast, crystalline AA had notably low stability. The process of pelleting of the diet had a different effect on the stability of the monitored vitamins. During storage, the stability of vitamin C and retinol was lower and α-tocopherol and β-carotene stability was higher in the pelleted feed than in the non-pelleted feed.