It is shown that a mixture of hard rods on aline in a homogeneous external field is ideal. The composition of such an adsorbed mixture is determined not by the relative values of the energies of the adsorbed molecules but by the relative values of a function similar to the enthalpy of a single molecule. In contrast, the hard rod mixture in an inhomogeneous external field is not ideal but displays minor negative deviations from Raoult’s law. It is suggested that the deviation from ideality depends significantly upon the dimensionality of the space.