The depth profiles of10B and11B 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 both10B and11B 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 ratioRp(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) for10B than for11B at a given energyEwhereSe∼Epandp≥0. The experimental data forRp(11B)/Rp(10B) andRp(11B) are compared with calculations, and it is demonstrated that the variation ofRp(11B)/Rp(10B) with ion energy hinges strongly on theSevsEdependence. A close velocity proportional dependence (p=0.50±0.03) is found to be valid up to ∼300 keV, and thenpdecreases gradually with a maximum inSe(p=0) at ∼2.0 to 2.5 MeV. A semiempirical expression is presented forSeand shown to yield excellent agreement withboththe relative isotope shift and the absolute range values; the deviations are less than 0.2% and 3.0%, respectively.