Abstract: A portable solar heating device is provided comprising an outer housing of transparent synthetic plastics material and a bladder within the outer housing. The bladder is of flexible sheet synthetic plastics material and has an expanded condition when filled with liquid and a collapsed condition when empty. The device has at least one port leading into the bladder, and means for minimising the area over which the bladder contacts the housing when the bladder is in its expanded condition, thereby to leave an airgap between the bladder and the housing over most of the surface area of the bladder.