A thermodynamic analysis was made of a set of two-component systems displaying isomorphous orientationally disordered phases (ODIC). The pure compounds are molecular materials belonging to the series (CH 3 ) 4– n1 C(CH 2 OH) n1 (n 1 =1, 2, 3), (NO 2 )(CH 3 ) 3– n2 C(CH 2 OH) n2 (n 2 =0, 1) and (NH 2 )(CH 3 ) 3– n3 C(CH 2 OH) n3 (n 3 =2, 3). The Equal Gibbs Curve method was used in order to determine the Gibbs excess energy of the orientationally disordered mixed crystals. The excess Gibbs energies of the disordered phases obtained from the disordered-liquid equilibria were used on the analysis of the low-temperature ordered-disordered equilibria. This excess property was correlated for the mentioned chemically coherent group of materials (neopentane derivatives) studied in this work, with a crystallographic parameter, the packing coefficient, which accounts for the steric factors and intermolecular interactions in the disordered mixed crystals.