In this paper, the reversible crystal-crystal transition of poly(hexamethylene dithiocarbonate) (PHMDC) has been systematically studied. According to various experimental results, we confirm that the transition shares the common features with Brill transition that is widely observed in nylons. This transition is clearly associated with the reversible trans-gauche conformational transformation of the methylene sequences in the crystal lattice. Therefore, in addition to its analogue poly(octamethylene carbonate) POMC (ACS Macro Lett. 2015, 4, 317-321), PHMDC can be considered as a new example of the polymers exhibiting Brill transition outside the nylon family. Careful examination of the experimental data reveals that for PHMDC the enthalpy ratio of the Brill transition to the melting is a constant of similar to 2/3, independent of the crystallization condition applied. Furthermore, the difference between the Brill transition temperatures determined from heating and cooling is also a constant, which is of similar to 4 degrees C. These new observations are obtained because the Brill transition of PHMDC is sharp and possesses a large transition enthalpy. Such a transition is associated with that the methylene segments in PHMDC can change their conformation between trans and gauche rapidly and completely in a narrow temperature interval. (C) 2017 Published by Elsevier Ltd.