Isomorphism and polymorphism in the binary chiral system of malic acid enantiomers (C4H6O5) were studied by means of X-ray diffractometry and high-temperature X-ray diffractometry. The ranges of solid solutions and quantitative ratios of phases were determined at room temperature in two types of binary systems: R-enantiomerRS-racemate of monoclinic modification M1 (13 compositions) and R-enantiomerRS-racemate of monoclinic modification M2 (15 compositions). Thermal deformations and polymorph transformations were characterized for crystalline structures of the S-enantiomer, modification M1 of RS-racemate, and modification M2 of RS-racemate (Р2, Р21/с, and Сс space groups, respectively). Parameters of the thermal deformation tensor were calculated and figures of thermal expansion coefficient projections on the ac plane of the crystalline structure were plotted for all the phases involved.