High‐efficiency diffractive elements with a graded volume distribution of refractive index are proposed, designed, and fabricated using thermal ion exchange in glass. A deliberate use of isotropic diffusion of ions enables us to generate, even with a single binary mask, a structure that acts like a continuous surface‐relief profile. The advantages of the proposed method include the ease of cleaning and antireflection coating the flat surface, the absence of alignment problems in the fabrication process, and compatibility with stacked planar optics. A grating beam splitter with three equal‐efficiency central orders and 89.7% diffraction efficiency is demonstrated. ©1995 American Institute of Physics.