Raman spectra of HNO(3)center dot NO(2) have been detected on liquid and solid surfaces in the presence of concentrated HNO(3) and NO(2) gas. The Raman spectrum of HNO(3) solutions containing N(2)O(4) has been partly reinterpreted in terms of contributions from HNO(3)center dot N(2)O(4) and N(2)O(4)center dot NO(3)(-) complexes.