1)3)-Introduction Studies of CdS thin films by using different techniques /1 to 5/ have gained importance recently because these films a r e used as window material in CdS/CuxS solar cells. Investigations have generally focused on the effect of thickness of films on the grain size. Studies on the effect of temperature /6,7/ have been rare. Also the research in this field has taken a newpath towards the possibility of using very thin CdS layers instead of thick ones ( e l 0 to 15 Mm) for the sake of production of cheaper cells /8/. But the use of thin films is hampered by their low efficiency a s well a s degradation caused by surface effects and grain boundaries. In the present note, this problem is investigated in the case of very thin CdS films. The relation between surface nature and the temperature of deposition of films is studied in this work.Theory The size factor of surface irregularities is calculated from the refractive index variation following the technique of Azzam and Bashara /9/ a s reported earlier /lo/. Here it is assumed that the film i s having a two-layer structure; a rough upper layer with its thickness equal to the r. m. s. value of the height of the irregularities and a lower layer with a perfectly smooth surface. Due to the presence of the rough upper layer, the measured value of the refractive index (N ) will be different from the actual value (Nf) of the material of the film. The relation between these two is expressed as /11/ e e ' 2 -= 9 7 9 N e + 2 N f + 2 where q is the volume factor of the surface irregularities.