Abstract: A method of increasing to a predetermined value the pressure exerted on materials treated in high pressure chambers. A first pressure-exerting force is used to act on the material throughout the entire treatment process and a second pressure-exerting force is used to act on a limited amount of a pressure compensating medium. The two pressure-exerting forces are interdependent whereby any tendency to change of the pressure exerted on the material inside the pressure chamber automatically brings about a proportional change of the pressure of the compensating medium, whereby the pressure in the high-pressure chamber is maintained at the desired level. The method is primarily but not exclusively intended for the production of synthetic diamonds.