In the paper, the heat stability was assessed of anthocyanins contained in the extracts prepared from blueberry fruit by heating them at 95 °C for 3 hours. In the samples, the following was determined: content of total anthocyanins, polymeric colour fraction, and colour of the extracts analysed. With the use of highperformance liquid chromatography coupled with mass spectrometry (HPLC/ESI-MS), the content of individual anthocyanins was identified and quantified. The heating process caused the total content of anthocyanins in the extracts to drop. The degradation process of anthocyanins went according to the kinetics of the first-order reaction. It was proved that a significant correlation existed between the content of anthocyanins and the L*value in the extracts analysed. This correlation suggests that the L*value may be a parameter suitable for quick monitoring the changes in anthocyanins in thermally processed products. In the paper, it was also shown that the individual monomers of anthocyanins had different heat-stability. The galactosides and glucosides were more resistant to heating compared to the containing arabinose.