In this paper a method is presented to measure the dielectric constant of materials. It is based on the analysis of the discontinuity problem of a circular cylindrical dielectric post centred in a rectangular waveguide. By expanding the field the reflection coefficient can be found. Curves have been made showing the relationship between the reflection coefficient and the dielectric constant of the post. For anisotropic materials the reflection coefficient will also depend on the direction of the cylinder. A rotation of the cylinder gives information on the dielectric constant in all directions. An experimental application is described to illustrate the procedure.