The efficiency of a semi-conductor detector is measured by compairing its response with that of an NaI(Tl) detector of known efficiency when both are exposed to the same flux of gamma-excited fluorescent X-rays. The experimental procedure is outlined and corrections and limitations are discussed. The technique is an inexpensive and convenient way of determining semi-conductor efficiency up to about 100 keV using a single radionuclide source of modest activity and readily-available fluorescers. © 1978.