Solid monolayers of binary mixtures of linear alkanes adsorbed on graphite are investigated at 10 K by X-ray diffraction. Monolayers of n-heptane and n-nonane,, which belong to the same 2D space group, mix continuously over the whole range of composition. In contrast, n-nonane and n-decane, belonging to different space groups have only limited miscibility. Interestingly, the isomorphous monolayers of n-octane and n-decane form a molecular compound. Such variety of the mixing behaviour is substantially different from that observed for the solid monolayers formed at the liquid-graphite interface.