The deep energy level of the isotope34S in the upper half of the energy gap of silicon is examined by isothermal transient capacitance measurements on ion‐implantation‐predeposited diode structures. The resulting energy level atEc−0.512 eV is found to be 0.014 eV closer to the conduction band edge than the corresponding deep level for the isotope32S in similarly prepared samples. The existence of an isotope shift for the deep sulfur level is interpreted as implying vibronic coupling between the electronic states of the sulfur center and the silicon lattice.