The objective of the study was to determine both the effect of temperature and of the time period of storage on the formation of hydroxymethylfurfural (HMF) in low-sugar jams from blackcurrant and highbush blueberries. The jams analyzed were stored for 180 days at a temperature of 22 ºC and 6 ºC. The range of the analysis conducted comprised the production of low sugar jams from berries; the determination of basic chemical composition of raw materials and jams; the assessment of the impact of storage conditions and temperature on the content of furfural, L-ascorbic acid, and sugars, as well as on the colour parameters of the jams analyzed. A method of high performance liquid chromatography for HMF was applied to determine the content of HMF, furfural, glucose, fructose, and sucrose, and a spectrophotometric method was used to determine the content of ascorbic acid. The measurement of colour was carried out in CIE L*a*b* system with the use of colorimetric method. In the jams analyzed, a significant impact of the temperature of storage was found on the formation of HMF and furfural. The highest increase in HMF occurred during the first two months of storage, especially, in the jams stored at a temperature of 22 °C. The content of sucrose was significantly impacted by the time period of storage, while the temperature of stored jams did not have any significant impact on the content of sugars. Based on the results of the correlation analysis, a linear dependence between the content of ascorbic acid and the content of furfural was reported. This dependence could be associated with the process of anaerobic degradation of this acid.