In this study, thermal performance of an air window collector having a horizontal black venetian blind was investigated experimentally under actual outdoor conditions. The venetian blind was installed between two 1.20 m by 0.80 m glass panes such that it can rotate about the horizontal axis. When it is fully open, it makes a 0° angle with the horizontal plane. The commercially available venetian blind made of aluminium slats and coated with black paint was used as energy absorbing surface. A fan is used to circulate the air in the system. Ambient air temperature, inside surface temperature of the test cell, collector inlet and outlet temperatures, inside surface temperature of inner the glass, temperatures of east, west, north side walls including floor and ceiling and solar radiation were measured and recorded using a data acquisition system. The effect of mass flow rate, separation distance between glass panes and energy absorbing slats and inclination angle of slats on the thermal performance of the air window collector were determined experimentally and presented in graphical form. It was observed that increasing mass flow rate increases the efficiency and increasing the separation distance decreases the efficiency.