The method of electrolytes leakage has been applied for identifi cation of distinctions between genotypes of a fl uffy oak (Quercus pubescens Wild.). This method allows revealing on level of electrolytes leakage distinctions between genotypes with high accuracy. Distinctions between genotypes become more obvious after application of temperature of a thermal shock which causes approximately 50% level of electrolytes leakage from their total content. It is revealed, that in comparison with the beginning of summer with approach of the hot period of july heat stability of leaves of an oak raise. Thus, change thermotolerance is used by plants as the adaptation to heats. Trees with late terms of blooming of leaves show heat stability in comparison with early dismissed. Apparently, accumulation of fat acids in cages of leaves at these plants occurs more vigorously, that is shown in change thermotolerance.