The effect of a superficial layer of paraffin or a plastic covering on the thermal storage efficiency of three kinds of non-saline solar pond has been investigated. Four small scale ponds were used for experimental measurements. These were: (1) a reference pond; (2) a pond divided horizontally into compartments separated by plastic sheets; (3) a pond with a superficial layer of paraffin and (4) a pond filled with hollow plastic cubes. Comparisons were made between these four ponds with and without plastic coverings, and with a layer of paraffin in addition on ponds 2 and 4. The rise in mean temperature of each pond during a number of heating cycles has been studied. A difference in mean temperature of the order of 10 to 20/sup 0/C was observed in each case between the beginning and end of the heating period and the temperature at the bottom of the pond after heating was in the range 30 to 50/sup 0/C. The results for these experimental ponds may be of use in the choice of full-size shallow ponds and may also give important indications for ponds of much greater depth.