Measurement of the intensity of light reflected from SiO2‐coated Si, compared with that reflected from the bare Si substrate, may be used for a SiO2thickness measurement in the range below 1000 Å where conventional methods are difficult to apply. By going into the ultraviolet much improved sensitivity is obtained, and at 200 nm an 18 Å film of SiO2can be detected. Relative reflectance curves for 589, 400, 300, and 200 nm derived from optical theory are presented; thicknesses measured using these curves are in good agreement with those from ellipsometry. The only apparatus required is a commercially available uv‐vis spectrophotometer equipped with a microbeam reflectance attachment operating at near‐normal incidence. Thickness measurement requires a spot 1–2 mm in diameter and is simple, rapid, and nondestructive.