The depth profiles of 10B and 11B implanted into amorphous silicon have been analyzed by secondary ion mass spectrometry. Implantation energies between 0.4 and 5.0 MeV were used, and each sample was sequentially implanted with both 10B and 11B without changing the acceleration voltage but only the field in the mass analyzing magnet. A shift between the two profiles is clearly resolved and has been carefully studied as a function of ion energy. A maximum shift of 3.5% in mean projected range (Rp) is revealed at 0.6–0.8 MeV [Rp(11B)≥Rp(10B)], and for higher energies the ratio Rp(11B)/Rp(10B) decreases slowly to a value of ∼1.006 at 5.0 MeV. This reverse shift (heavier isotope penetrates deeper) is attributed to a larger electronic stopping cross section (Se) for 10B than for 11B at a given energy E where Se∼Ep and p≥0. The experimental data for Rp(11B)/Rp(10B) and Rp(11B) are compared with calculations, and it is demonstrated that the variation of Rp(11B)/Rp(10B) with ion energy hinges strongly on the Se vs E dependence. A close velocity proportional dependence (p=0.50±0.03) is found to be valid up to ∼300 keV, and then p decreases gradually with a maximum in Se (p=0) at ∼2.0 to 2.5 MeV. A semiempirical expression is presented for Se and shown to yield excellent agreement with both the relative isotope shift and the absolute range values; the deviations are less than 0.2% and 3.0%, respectively.