Introduction. Incomplete cross-linking of composite dental materials leads to their susceptibility to degradation in the environment of non-organic and organic solvents, contributing to the release of chemical compounds which are potentially harmful to living organisms. Objective. The aim of the study was an evaluation in in vitro conditions of releasing of potentially toxic substances from six dental composite materials available in EU countries. Materials and methods. The following compounds released from the samples stored in water were analyzed: bisphenol A (BPA), triethylene glycol-dimethacrylate (TEGDMA), urethane dimethacrylate (UDMA) and ethylene glycol dimethacrylate (EDGMA). Analysis of the substances was performed with the use of high performance liquid chromatography, after the following incubation periods: 1 hour, 24 hours, 7 days and 30 days. Results. Among the analyzed substances, after 1 hour of incubation, the highest average concentration was found for TEGDMA – 2045 μg cm-3 (in Herculite XRV material), after 24 hours – for UDMA 4.402 μg cm-3 (in Gradia Direct Anterior material) and after 7 and 30 days for TEGDMA: 8.112 and 6.458 μg•cm-3 respectively (in Charisma material). Conclusions. The examined composites used for reconstruction of hard tissues of teeth remain chemically unstable after polymerization, and release potentially harmful substances in conditions of the present study. The dynamics of the releasing of potentially harmful substances is correlated with the period of sample storage in water.