The formation of HNO3.(H2O)(n) complexes is studied both theoretically and experimentally. First principles electronic structure calculations were used to produce minimum structures and harmonic vibrational frequencies of HNO3.(H2O)(n) and DNO3.(H2O)(n) complexes (n = 0-4). They also provide insight into the ionization of HNO3 in water, predicting that ionization in isolated clusters occurs for n = 4 or larger. Vibrational absorption spectra of matrix isolated nitric acid/water complexes were obtained using an FTIR spectrometer-based instrument. By incrementally increasing the amount of H2O in the matrix, we have been able to study nitric acid/water complexes and the ionization of HNO3. New spectral assignments, the first ones for the n = 2 and 3 complexes, are suggested on the basis of the results of the electronic structure calculations.