DSC measurements and microscopic observations of three compounds from the thioester series (4-n-pentylphenyl-4 -n-alkoxythiobenzoates, abbreviated to <S5) have been performed.In addition to the nematic and smectic phases known previously from the literature, a new monotropic low-temperature phase has been detected by both methods for 9S5 and 10S5. This new phase exists below a highly ordered smectic J phase and a crystal phase. The nature of this new phase is discussed.