The two‐dimensional, scalar diffraction theory is developed for a diffusing surface as suggested by Schroeder, consisting of an array of ’’wells’’ of equal widths but different depths, separated by hard, thin walls. Diffusors of finite size as well as infinite, periodic ones are treated. Matching the plane waves in the free half‐space to the wave modes in the wells yields an infinite but discrete linear equation system in either case. For a periodic diffusor based on a quadratic‐residue (QR) sequence, numerical results are compared with measurements on a finite diffusor comprising two periods. The results are also compared with those obtained by approximating the diffusor as a locally reacting surface. The differences are, at most, a few decibels; the mode‐matching results are usually closer to the measurements. The uniformity of scattering is tested over a range of wavelengths of 1:4; QR sequences are superior to pseudorandom sequences.