The berries of black currant and black aronia are rich in polyphenolic compounds and especially in anthocyanins, demonstrating antioxidant activity. The aim of the study was to evaluate the possible effect of thermal technological processes on the quantity of polyphenols and anthocyanins in berry juice concentrates, and on the antioxidant activity. After 8 hour storage of black currant and black aronia berry juice concentrates at 60 degrees C, the amount of polyphenols decreased by 46% and 22%, anthocyanins 31% and 35%, respectively. Antioxidant activity decreased by 26% and 56%, respectively. The results demonstrated insufficient stability of juice concentrates, and impropriety of application of long lasting drying processes in manufacturing of black currant and black aronia berry dry products. Fast and efficient drying methods for liquid products should be applied to preserve qualitative and quantitative composition and their antioxidant activity.