[en] The threshold behaviour of the cross-section for cations formed via electron impact ionization to methane (CH₄, CH₃D and CD₄), ethane (C₂H₆) and propane (C₃H₈) has been studied in a crossed electron/neutral beams apparatus with mass spectroscopic analysis of the ions. The appearance energies of the ions formed via electron impact ionization to these molecules were determined at room and at elevated temperatures (about 700 K). At elevated gas temperatures a decrease of the appearance energies has been observed for all molecules (red shift). The red shift appears to depend on the size and on the isotopic composition of the molecules. These results indicate that at very high gas temperatures the cross-sections for electron impact ionization in the threshold region changes may substantially differ from the low temperature one. Additionally, differences in the appearance energies due to isotopic substitution have been observed for molecules CH₄, CH₃D and CD₄. (author)