Water/air mass stopping power ratios have been calculated for 5, 10, 20 and 30 MeV electron beams and various photon beams from 60Co to 31 MV betatron. The stopping power ratios have been evaluated by applying a modified version of the Spencer-Attix theory to depth-dependent electron flux spectra computed by the Monte Carlo method. Results are presented for three values of the cavity size parameter delta, 0.001, 0.01 and 0.1 MeV, and also for a Bragg-Gray cavity. For the electron beams, the results are compared to Berger's comprehensive computations. In the case of the photon beams, such a rigorous evaluation of the stopping power ratio has not been carried out previously. It is shown that the currently used approximate Bragg-Gray ratios are as much as 2% too low for high energy photon beams, and that there is a difference of about 1% between values for betatron (thin target) and linac (thick target) beams of the same maximum photon energy.