Enantiomers (R(+) and S(-)),RS-racemate (double compound) and (R+S)-conglomerate (mechanical mixture of enantiomers) of malic acid C 4 H 6 O 5 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 P2 1 /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 found to depend on the type of crystallization medium (a melt,aqueous medium,ethanol or acetone solution),crystallization rate (from 2-3 minutes to 4 months),and crystallization temperature.Heating S-enantiomer and M1 RS-racemate up to their respective melting points (100 ℃ and 124 ℃,correspondingly) only made them undergo thermal deformations,while heating (R+S)-conglomerate in the temperature range of 96-110 ℃ resulted in its homogenization to form M2 RS-racemate,which,near the melting point (118 ℃),namely,in the range of 112-116 ℃,was transformed into M1 RS-racemate. Enantiomers (R (+) and S (-)), RS-racemate (double compound) and (R + S) -conglomerate (mechanical mixture of enantiomers) of malic acid C 4 H 6 O 5 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 P2 1 / c), M2 (monoclinic, space group Cc ), and Tc (triclinic, space group P-1), the latter has been previously unknown. Modification Tc was also described, and its Xray diffraction characteristics, including interplanar spacings d, hkl indices, unit cell parameters, were defined. 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 + T c) was found to depend on the type of crystallization medium (a melt, aqueous medium, ethanol or acetone solution), crystallization rate (from 2-3 minutes to 4 months), and crystallization temperature. Heating S-enantiomer and M1 RS- racemate up to their respective melting points (100 ° C and 124 ° C, correspondingly) only made them undergo thermal deformations, while heating (R + S) -conglomerate in the temperature range of 96-110 ° C resulted in its homogenization to form M2 RS- racemate, which, near the melting point (118 ° C), namely, in the range of 112-116 ° C was was transformed into M1 RS-racemate.