Mixing behaviour in adsorbed layers at the solid-liquid interface depends upon both the symmetry and the quantitative similarity of the unit cells of the two pure monolayers. Here we present data that demonstrates that this similarity can be conveniently expressed in terms of a 2D isomorphism coefficient and identify values of this coefficient that delineate regions of immiscibility, partial or complete miscibility. Here we have established the utility of this coefficient for mixtures of linear alcohols and mixtures of linear alkanes adsorbed on graphite but expect that these rules should be as general in their applicability to 2D layers as are similar rules for bulk phases. (C) 2003 Elsevier Science B.V. All rights reserved.