Relevance of the work is determined by the necessity of studying the thermal behavior of Poisson's ratios n in potassium halogenides KX (X=F, Cl, Br, I). The detailed research will allow filling the void in the scientific knowledge concerning the given objects. The aim of the research is to obtain the Poisson's ratios of КCl, КBr, КI mono- and polycrystals for temperature region of their existence and those of KF under Т£300 К; to identify the regularities between anisotropic Poisson's ratios n, shear moduli G and Young moduli Е for potassium halogenides KX (X=F,Cl,Br,I). Methods of research. Based on the known experimental values of stiffness constants сij the authors have determined the anisotropic and isotropic (for polycrystals) Poisson's ratios of potassium halogenides KF, КCl, КBr, КI. Results. The Poisson's ratios for mono- and polycrystals of КCl, КBr, КI were found for temperature region of their existence and for KF under Т£300 К. On the example of potassium halogenides the authors proved the inequations between the anisotropic Poisson's ratios n>n>n>n, shear moduli GЕ>Е previously stated for lithium and sodium halogenides (up to the temperature points of elastic isotropy). It was established that with the temperature growth two anisotropic Poisson's ratios (n, n) in potassium halogenides decrease, two other (n, n) increase, the average one (n) demonstrates weak temperature dependence (insignificant growth). It was established that all anisotropic shear G and Young Е moduli in KX crystals decrease as crystal temperature growth.