Semi-empirical MNDO calculations are employed to study the protonation of chlorine nitrate, a molecule of increasing importance to the chemistry of stratospheric ozone depletion. Predictions about structural changes and charge redistributions are made. A value of 159 Kcal mor for the proton affinity of chlorine nitrate is obtained. An interesting forced dissociation of protonated chlorine nitrate into HOCI and NO +2 IS reported.