The sensitivity of the N-14-hyperfine splitting of nitroxide spin to the local electric field, E-loc, has often been used to probe the polarity of microenvironments; however, two uncertainties have been faced at once as follows : (i) the value of E-loc in a complex microenvironment and (ii) the effect of E-loc on the hyperfine splitting. In this paper, we study nitroxide free radicals that may be charged (positively or negatively) by varying the pH. The additional charge produces an additional E-loc, whose value we may calculate with confidence, allowing us to focus attention on the effect that E-loc has on the hyperfine splitting. A simple theory is developed that provides a link between the shift in the N-14-hyperfine splitting, Delta A(N), and the local electric field, E-loc. To test this approach, we have calculated the Delta A(N) for several charged nitroxide spin probes and found good agreement (both magnitude and sign) with the measured values using Delta A(N) = -3.8 x 10(-8)E(loc) (Delta A(N) in Gauss and E-loc in V/cm), where E-loc is the component of E-loc directed along the N-O bond from the nitrogen to the oxygen.