The geometries of the lowest stationary points on the triplet and singlet 'NO3+' potential energy surfaces are investigated. It is found that the skewed T-shaped NO+ . O-2((X) over tilde(3)A ") state has the lowest energy, with a linear ((A) over tilde(3)Sigma(-)) minimum lying similar to 1 kcal mol(-1) higher in energy. About 1 eV higher in energy lie the cis and trans isomers of the closed-shell <(alpha)over tilde>(1)A' state, with the cis state being lower in energy. Above the <(alpha)over tilde> state ties the D-3h (X) over tilde(1)A'(1) state of NO3+ - the cation of the nitrate radical. It is found that the MPn series is rather oscillatory; for NO3+ and so is probably unreliable. The binding energy, enthalpy and and compared to available values. Ionospheric entropy of complexation of the (X) over tilde(3)A " NO+ . O-2 complex are calculated implications are noted.