The paper describes a precise method of determining the errors of flatness of a 30 cm diameter interferometric reference surface, in which full account is taken of the deflection of the flat due to gravitational influence, and it is shown that special precautions must be taken to avoid spurious errors caused by small temperature gradients in the environment. The results quoted include contour maps showing the sag of circular flats which are supported at three points equally spaced round the periphery. Experiments with liquid reference surfaces indicate that, under suitable conditions, a flatness approaching ±1/100 fringe can be obtained.