The algebraic luminance difference corresponding to a uniform achromatic surface on a uniform achromatic background determines the dark or light quality and the extent of this quality in the achromatic colour of the surface, while the absolute value of this difference determines the amount of perceived contrast between the achromatic colours of the surface and of the background. The results of an experiment designed to test whether grouping of achromatic surfaces depends on one or on both of these differences are here reported. Grouping was found to depend prevailingly on algebraic luminance difference in some subjects, prevailingly on absolute luminance differences in other subjects, and on both of these differences with similar probabilities in still other subjects.