Weak satellites of the strong and sharp parity-forbidden, but symmetry-allowed 1s(A1)→1s(T2) doublet of the S+ donor spectrum in silicon are reported. The low-energy ones are ascribed to a Si isotope effect and the high-energy ones to an S isotope effect. We also show that the profile of the low-energy component of the same transition of Se+ in silicon can be reproduced from the computed combination of Si and Se isotope shifts with the same characteristics as those of S+, but with a smaller Se isotope shift. The Si isotope shift is discussed in terms of the bond softening effect in the electronic ground state for a cluster involving the chalcogen atom and its first nearest neighbors. The positive chalcogen isotope shift is discussed by considering the effect of a vibronic coupling to the τ2 mode of vibration in the excited state within the bond softening framework. It is also shown that this vibronic coupling can account for the noneffective mass behavior of the 1s(T2) level.