The invention comprises liquid crystalline polyesters suited to be used as a medium for optical data storage, especially reversible (or erasable) optical data storage. The polyesters comprise the polycondensation product of an aliphatic dicarboxylic acid or a derivative thereof and propane diol which on its center carbon bears a mesogenic substituent comprising two rings, either alicyclic or aromatic. Preferably also the dicarboxylic acid or derivative thereof is mesogenically substituted. The invention further comprises a medium for optical data storage containing polyesters of the above type.