A pressure compensation system for enclosed spaces at a subterranean location changes volume with thermally induced solubility changes of a salt in water. The salt is held in an enclosure that is either rigid, or impervious and flexible or porous and flexible. As well conditions change and temperature increases, some of the salt goes into solution with a resulting decrease in volume that compensates for thermally induced volume increase due to temperature increase in the borehole. Conversely, a decrease in borehole temperature brings some of the salt out of solution for a volume increase to offset the volume decrease of the adjacent fluid to keep the pressure stabilized in the enclosed volume. In the porous enclosure embodiment the openings are sufficiently small to retain the salt even in solution. However, minimal net flows are anticipated for pressure compensation due to changing thermal effects.