Binary mixture of 4 octyloxy- 4′-cyano- biphenyl (80CB) and 4-n-hexyl phenyl 4-n′ pentyloxy benzoate (ME 50.6) shows enhanced smectic A phase, where 80CB has both nematic and smectic A phases and ME 50.6 has only nematic phase. In the present paper we report the phase diagram, refractive indices, densities and orientational order parameters of different binary mixtures of 80CB/ME 50.6 throughout the entire composition range. Birefringence as well as density values show minima with molr fraction of 80CB. Order of smectic to nematic phase transition has been discussed.