The effect of a solid solution of hydrogen, oxygen, nitrogen, or water vapor in the lattice of thin films of Al was studied by x-ray diffraction. The lattice parameter shift due to solid solution effect independent of residual strain or faulting was observed. This method is applied to thin films to determine nondestructively whether there are low concentrations of impurities affecting the lattice of the films. It is also shown that large errors occur in the value of the residual stress measured by certain x-ray techniques or in that of the solid solution concentration if both of these effects are not taken into consideration during the subsequent calculations.