Natural convection, radiation and conduction heat transfer on horizontal concrete slab systems is experimentally studied. The system was a 0.78 m long, 0.40 m. wide and 0.10 m thick concrete slab. A heat source was used to impose a constant heat flux which could be varied from about 200 to 700 W/m(2). Temperatures at various points and heat flux by natural convection at the horizontal surface were measured. Using various assumptions, the system was also analyzed theoretically. It is found that the mathematical model to study the transient heat transfer in the slab system was satisfactory to predict its thermal behavior in various conditions, An empirical correlation for natural convection on the horizontal concrete slab was derived and used in the analysis. The results showed that the incident energy on the concrete slab was not a parameter affecting strongly the absorbed heat by the slab, the radiation heat losses made about 60% while those by natural convection 40%, and the major energy storage-restitution was taking place during the first 3 to 4 h. The optimization of energy storage density and the thermal performance was also discussed and various parameters affecting them were defined. (C) 2002 Elsevier Science Ltd. All rights reserved.