We present measurements of the spectrum (1-80 MHz) of the effect of a weak
(similar to 500 muT) radio frequency magnetic field on the electron-hole re
combination of radical ion pairs in solution. Distinct spectra are observed
for the pyrene anion /dimethylaniline cation radical pair in which one or
both of the radicals are perdeuterated. The radical pair mechanism is devel
oped theoretically and shown to account satisfactorily for both the magneti
c field effect and the associated magnetic isotope effect.