alpha,omega -bis(4-n-alkylanilinebenzylidine-4'-oxy)alkanes exhibit a rich smectic polymorphism. We have studied the variation of density as a function of temperature across the phase transitions in two compounds of the series viz., 7.040.7 and 7.050.7. Both of these compounds exhibit a rare phase transition of Sin A - Sin F. The results confirmed first order nature of the transitions of Isotropic - Sm A and Spa A - Sm. F, but with a smaller jump than expected in density as compared to monomers across the transitions. The estimated pressure dependence of transition temperature is also reported. The results are discussed in the light of available literature data.