Enantiomers (R(+) and S(−)), RS-racemate (double compound) and (R+S)-conglomerate (mechanical mixture of enantiomers) of malic acid C4H6O5 have been investigated by means of X-ray diffractometry and high temperature X-ray diffraction method. The RS-racemate was found to be able to form three polymorphic modifications, which we denominated as M1 (monoclinic, space group P21/c), M2 (monoclinic, space group Cc), and Tc (triclinic, space group P-1), the latter modification having been unknown before. Modification Tc was also described, and its Xray diffraction characteristics, including interplanar spacings d, hkl indices, unit cell parameters, were defined. In addition, X-ray diffraction characteristics for both reported earlier M1 and M2 monoclinic polymorphic modifications were measured with higher accuracy. The ability of RS-racemate to form one of the above three modifications (M1, M2, and Tc) or their mixtures containing various proportions and combinations of the components (M1+M2, M1+Tc, or M2 +Tc) was fo