Time-resolved EPR and phosphorescence spectra have been observed for the lowest excited triplet states of 4-nitropyridine N-oxide and its alkyl derivatives. It has been shown that the ratio of the zero-field-splitting (zfs) parameters reflects the molecular geometry : the E/D value decreases with increasing the twisting angle of the nitro group from the molecular plane. Protic solvents induce significant changes in the zfs parameters and the population ratio of the triplet sublevels. The results are discussed in terms of formation of the hydrogen bonding complex at the N-oxide oxygen atom.