In electronic packages where high voltages exist, a high dielectric strength environment surrounding the components is necessary. Liquid-filled units have used various oils which in addition to dielectric strength provide good thermal paths for cooling; evaporative-cooled units often use refrigerants. In either case allowance, volume-wise, must be made for thermal expansion of the fluid. A simple, novel, weight-saving technique for accomplishing this consists essentially of replacing the conventional container with a flexible one (for example, fabric-reinforced rubber). The flexible container then expands and contracts accomodating volume variations in the fluid. Design details, test results, and a general evaluation illustrating the adaptability of the flexible container as a practical design tool are discussed.