Mixed-layer illite/smectite minerals from the Dolna Ves hydrothermal deposit were studied. Expandability ranged between 45 to 6 %. Two types of interstratification R1 and R3, were observed, and only 1M polytype was found. The variability in chemical composition of the I/S minerals was small. Layer charge is mainly tetrahedral. Mixed-layer I/S minerals were formed by reaction between K-bearing hydrothermal solutions and smectites. A zonation in expandability was observed in the studied area. It decreases from the south to the north and is related to decrease of the temperature of hydrothermal solution coming from the north. The solutions probably had temperatures that ranged between 120 to 200°C.