Abstract: X-ray diffraction (XRD) analysis of small quantities of clay mounted on glass slides using conventional Bragg-Brentano geometry generally produces unsatisfactory low-intensity reflections masked by amorphous substrate scatter. Glancing-incidence asymmetric Bragg diffraction, an alternative uncoupled geometry, uses a fixed low-incidence angle and parallel-beam optics to increase path length through the sample and decrease X-ray penetration into the substrate. To evaluate this technique on thin soil clay films, results from conventional Bragg-Brentano and glancing-incidence diffraction (GID) were compared for progressively diluted clay suspensions separated from 2 southeastern soils with typical mineral assemblages. Patterns produced by GID showed overall higher reflection intensities and reduced substrate scatter, especially at higher 20 angles within the amorphous glass region. Using GID, positive identification of clay minerals was obtained from sample quantities as small as 0.005 mg cm−2 and suspensions as dilute as 29 mg L−1.