This paper presents experimental and theoretical studies of a hat plate solar air heater. The single pass heater was constructed locally and tested in southern Iraq. The upper face of the rear plate was then painted matt black, and tests were repeated to investigate the influence of thermal radiative exchanges inside the air channel. Various parameters, such as temperature, solar intensity and air flow rate, were recorded and used to plot performance curves. A heat transfer model, based on the subdivision concept, was developed and solved by computer. The predicted transient variation of outlet, air temperature compared reasonably well with measured values. The present work shows that the efficiency of solar air heaters can be improved by painting the rear plate black; an increase of up to 10% is reported.