Thermoluminescence (TL) glow curves of polycrystalline tin dioxide samples exhibit three main peaks below room temperature. Their relative intensities depend on experimental conditions. Especially, they are modified by aluminum doping or by wet air exposition. In SnO₂, we suggest that ionised oxygen vacancies and hydroxyl groups are responsible for thermoluminescence. This assumption is discussed. In order to explain the behaviour of these defects, two kinds of modelling are proposed.