X-ray production by proton bombardment may be used to determine the surface density of thin films. It is desirable that the energy loss of the protons in the film be small and that the self-absorption of the produced x-ray be negligible. To illustrate the method, thin surface films of aluminum are discussed. The aluminum x-ray yield (x rays/proton) as a function of surface density is presented for the range 0 to 115 μg/cm2 and at an incident proton energy of 100 keV. Under the conditions of a 10-μA proton current incident on a thick target, a typical detection system would record 10 000 counts/sec with a counter signal-to-noise ratio of 104. At a signal-to-noise ratio as low as unity, the calculated surface density is 0.005 μg/cm2 or approximately 1014 atoms/cm2. Details of the calculation are presented, along with a table of eleven elements to which the method is currently applicable.