X‐ray production by proton bombardment was used to determine the surface density of thin films of Al, Cu, and Yb. This was accomplished by comparison at a number of thicknesses with known films. In addition, the x‐ray yield for Al was compared with values predicted by the use of x‐ray production cross section, stopping power, and bulk density. The experimental and calculated values agreed within 15%. The data are presented in linear form from 0 to 1900 Å and semilogarithmic form from 0 to 7000 Å, which is nearly a thick film for this technique. The useful upper limit lies in the range of 1000 Å for Yb and 4000 Å for Al. The lower limit is not clearly established, although the measurements for the Al surface densities were limited to surface densities equivalent to 1.2 Å as established by substrate contamination.