Four kinds of Si(Li) and a high‐purity Ge x‐ray detectors have been tested to measure the thicknesses of gold contact layers by comparing the response of the primary Rb‐K&agr; x‐rays with that of the induced Au‐L&agr; x rays. Observed thicknesses ranged from 140 to 260 A˚. Near the x‐ray energies of Au‐Msubshell absorption edges, the contribution of x‐ray transmission through the observed gold layers to the detection efficiency is greater than or equivalent to that of the inherently attached 7.6‐&mgr;m beryllium window. Hence, one must be careful in the determination of the detection efficiency in x‐ray semiconductor detector when low‐energy x rays of less than about 4 keV are measured.