The analysis of thermohaline properties of the sea water in the Piran Bay is based on 17-year data of the following parameters: temperature, salinity and density of the sea water. Used for the investigation was the complete water column, or, the values of weight averages were used. The data of the 1973-1989 period were studied, by seasons. Most of a year, the Piran aquatory acts as an autonomous thermohaline unit within the frame of the Trieste Bay. This dependence can be blurred occasionally due to the influence of external and internal factors. The thermohaline properties of the sea water in the piran Bay are primarily influenced by the continental and the hydrological factors, and only secondary by the marine factors.