Summary: Thin-section analysis is essentially an areal measurement, the measurement area usually being the upper surface of the section. If transmitted light is used for the measurement, the apparent areas of opaque grains will in general be somewhat larger than their true areas on the measurement surface. For strictly spherical opaque particles in a transparent matrix the expected excess of apparent over true area is shown to be (πr²k)/(2r + k) where r is the spherical radius and κ is the thickness of the thin section. A table shows the relation between true and apparent area as a function of r/k.