A sample cell in a polarimeter comprises a thermally conductive body enclosing a substantially cylindrical internal volume with an insulating outer cover which leaves one face exposed for complementarily contacting a temperature controlled base plate so as to keep the sample cell at a predetermined temperature, and a means for positioning the sample cell at a predetermined position such that a measuring light beam longitudinally passes through the sample cell. The water jacket system required for the conventional cylindrical sample cells is eliminated. A cell holder for holding a cylindrical sample cell in a polarimeter comprises a heat transfer element having a concave cylindrical heat transfer surface for complementarily contacting an external cylindrical surface of the cylindrical sample cell so as to keep the sample cell at a predetermined temperature. The heat transfer element is thermally conductive to a thermal electrical conductor element the temperature of which is controllable by an electric current. The cylindrical sample cell is positioned in the light path of the polarized light beam by a cell mount formed by the heat transfer element, or by a pair of flanges resting on a pair of parallel rails. The invention eliminates the use of the water jacket temperature control system for the cylindrical sample cells.