The symmetric liquid crystal dimer homologous series, alpha,omega-bis-(4-n-alkylaniline benzylidene-4'-oxy) alkanes, show a rich variety of smectic polymorphism. We report here the nature of different transitions exhibited by two of these dimers using density measurements as a function of temperature. The compound 6.O12O.6 exhibits only nematic phase; while 7.O6O.7 exhibits smectic A and smectic F phases. All the phase transitions studied viz., isotropic nematic, isotropic smectic A, and smectic A - smectic F are confirmed to be of first order nature. The calculated density jumps, thermal expansion coefficient data and pressure dependence of transition temperatures are analysed in the light of available literature data.