The gas-phase clustering reactions NO 2 −· (C 2H 5ONO 2) n−1 + C 2H 5ONO 2 = NO 2 −· (C 2H 5ONO 2) n ( n = 1–3) NO 3 −· (C 2H 5ONO 2) n−1 + C 2H 5ONO 2 = NO 3 −· (C 2H 5ONO 2) n ( n = 1 and 2) have been studied by means of a high-pressure mass spectrometer at temperatures from 200 to 380 K and at total pressures of 0.5–2 torr. It is demonstrated that under the conditions used here these reactions, with the exception of NO 2 − + C 2H 5ONO 2 = NO 2 −· C 2H 5ONO 2, achieve thermodynamic equilibrium above 1 torr. On the basis of the temperature dependence measurements of the equilibrium constants, thermodynamic data have been determined. The following Δ H n−1 0, n values were obtained: NO 2 −·(C 2H 5ONO 2) n ; Δ H 0,1 0 =−20.9 (indirectly obtained), Δ H 1,2 0 = −8.5 and Δ H 2,3 0 = −7.3 kcal mol −1; NO 2 −·(C 2H 5ONO 2) n ; Δ H 0,1 0 = −17.2 and Δ H 1,2 0 = −7.2 kcal mol −1. Semiempirical INDO MO calculations were performed for NO 2 −· (C 2H 50NO 2) n = 1,2 and NO 3 −·(C 2H 5ONO 2) n = 1,2 to test the structures and binding energies of these systems. The experimental observations are discussed in terms of the structure stabilities of the solvated ions.