Photothermal,deflection spectroscopy is used to determine optical absorption in ZnS:Mn thin film electroluminescent phosphors. The ZnS:Mn sample is immersed in a transparent liquid and optically pumped with a square-wave intensity modulated monochromatic light source. This produces a thermal gradient in the fluid which is probed by a He-Ne laser beam grazing the sample surface. The optical absorption in the sample can be related to the deflection of the laser beam by the thermal lens in the fluid. The relation of the absorption to the energy levels in ZnS:Mn will be discussed.