The thickness of a film growing during one oscillation period at MBE of silicon and germanium on a slightly misoriented Si(111) surface was measured as a function of temperature. The temperature elevation was found to result in either an increase or decrease in the oscillation period. That was accounted for by the occurrence of various barriers for incorporation of adatoms at the upper terrace into differently directed steps. The activation energies of nucleation of two-dimensional islands were determined. In the case of a decrease in the oscillation period the activation energy was 1.45 +/- 0.1 eV and 1.1 +/- 0.1 eV for epitaxy of silicon and germanium, respectively. In the case of a increase in the oscillation period the activation energy was 0.91 +/- 0.1 eV and 0.49 +/-0.1 eV for epitaxy of silicon and germanium, respectively.