The depth of penetration of 0.5‐ to 11‐kev electrons and 1‐ to 25‐kev H+, H2+, and He+ions in aluminum has been measured. It has been found that the Thompson‐Whiddington law for electrons does not hold for electrons having energies less than 10 kev. The practical range‐energy relation obtained for electrons in aluminum was found to beR = KE1.3, whereKis 0.042 ifEis expressed in kev andRin microns. The practical range‐energy relation for light ions in aluminum was found to beR = KE0.83whereK= 0.020 for H+,K= 0.021 for He+, andK= 0.015 for H2+ifEis expressed in kev andRin microns. Results are compared with those obtained by previous observers.