The aim of the study was to examine the influence of rapid high-intensity polymerisation on water sorption and solubility of contemporary bulk-fill composite materials. Five composite materials were tested: four bulk-fill materials, Filtek One Bulk Fill (3M, St. Paul, USA), Tetric PowerFill (Ivoclar Vivadent, Schaan, Liechtenstein), Tetric PowerFlow (Ivoclar Vivadent), SDR (Dentsply, Konstanz, Germany), and one conventional composite (Filtek Z250, 3M). Composite specimens, 9 mm in diameter and 2 mm high (n = 6), were polymerized using an LED curing unit (Bluephase® PowerCure, Ivoclar Vivadent). In the standard group, specimens were polymerised for a total of 40 s on both sides (1193 mW/cm2) and in the 3s group for 3 s on one side (3053 mW/cm2). Water sorption and solubility were measured by the ISO 4049 method during 30 days of immersion. Results were analysed with one-way ANOVA with Tukey’s adjustment, and a t-test compared different polymerisation protocols for the same material (p<0.05). The three-second polymerization protocol affected the water sorption and solubility of almost all materials. Filtek One Bulk Fill showed significantly higher water sorption and solubility, while Tetric PowerFlow proved to be a stable material with low values of tested parameters. Sample saturation during immersion was achieved in low-viscosity bulk-fill composites after only two weeks, while in other materials it was not achieved even after 30 days. The effect of rapid polymerization on water sorption was material dependent. However, the three-second polymerization method caused an increase in solubility, which can be attributed to the formation of short-chain oligomers and the elusion of unreacted monomers.