This paper presents the results of macro scale tests that compare the thermal storage performance of ordinary concrete blocks with those that have been impregnated with two phase change materials (PCM). One is a commercial Butyl Stearate (Emerest 2326), and the other is a commercial Paraffin (Unicere 55). The comparative characteristics of these PCM - concrete combinations were examined. Also, the effect of air velocity was studied in respect to the control of the rates of heat storage and discharge. This research is an extension of the laboratory scale work in this area, which were carried out in recent years. (C) 2000 Elsevier Science B.V. All rights reserved.