This research document presents an experimental study on the effect of the superposition of glass panes on the functioning of a solar thermal collector. Experiments were carried out on an active solar energy demonstration system (ET 200). It consists of a flat plate solar collector, water storage tank, a control and command cabinet and a highpower lamp, simulating natural sunlight. Three (03) commercial glass panes of 3mm, 5mm and 8mm thick were used. In the first experiment the collector glass was replaced by the superposition of the two panes of 3 and 5mm thick. In the second experiment the collector glass was replaced by the thickest glass (8mm). The glass of 8mm thick leaves more energy, generated from the halogen lamp, to penetrate inside the collector than the composed glass (3mm+5mm), which leaves less energy reaching the absorber. The glass pane of 8mm thick has an efficiency of about 44% against 40% for the composed glass (3mm+5mm). The glass composed from the superposition of the panes of 3mm and 5mm thick exerts a distinct effect, on the solar collector, than the simple glass of 8mm thick. So, it is not possible to replace the single pane of 8mm thick by the superposition of the two panes of 3 and 5mm thick. 