A thermal compensating apparatus method for maintaining a substantially constant fluid pressure within a subterranean well tool of the type that includes a bladder that is selectively expandable upon the introduction of pressurized actuation fluid for actuating said tool at a location in a well. A body includes first and second fluid chambers. The first fluid chamber houses a substantially incompressible fluid and communicates with the actuating fluid used for activating said tool in the well. The second fluid chamber is charged with a compressible fluid. Both chambers define first volumetric sizes within the body upon actuation of said tool in the well. The fluid chambers are operatively connected to each other without transmitting fluid there between so that changes in the volumetric size of the first chamber caused by temperature variations in the actuation fluid will change the volumetric size of the second fluid chamber for maintaining the actuating fluid at a substantially constant pressure.