A technique utilizing the reflection of x‐rays to determine material density at flat surfaces is described. The effects of sample misalignment limit the accuracy of x‐ray reflectivity as typically practiced. These effects may be properly accounted for by measuring the critical angle for reflection at many different x‐ray wavelengths simultaneously from which an extrapolation of the position of the critical angle at infinite wavelength may be made. This extrapolation has the effect of correcting for sample misalignment. Use of the technique is demonstrated for single‐crystal silicon surfaces and for silica spin‐on‐glass thin films. ©1995 American Institute of Physics.