Boron oxide-based systems on structure and properties are similar to silicate systems, but they are more fusible and so are widely used in modelling various metallurgical processes. This paper presents the results of viscosity, electrical conductivity, surface tension and density measurements of the B2O3–CaO system with a content of 25–45% CaO in the temperature range above the liquidus temperature. To measure viscosity, vibration viscometry was used. Electrical conductivity was measured via the contact method using an alternating current bridge. Surface tension and density were measured using the lying drop method. The obtained results were used to describe the structure of borate melts.