An original thin optical cell for high pressure studies is described. The system of back pressure employed prevents the two windows from moving. In this way the only limitation in the control of thickness is due to elastic deformation of the two windows under pressure. The cell is particularly convenient for measurement of shifts of intense absorption bands. An example is given in the case of atomic lines of xenon in the region 1150–1500 Å, where thicknesses of less than 1 &mgr; are necessary.