Abstract Using in-situ microbalance and infrared spectroscopy techniques the double ammonia proton complexation was traced. The results confirm the formation of N2H7+ dimer in solid Dawson acid H6P2W18O62, previously reported only for N2H7I and for (N2H7)4SiW12O40. The formation of such dimers was evidenced by the microbalance results, the molar ratio of ammonia to proton was measured as 2:1 at 10.7 kPa and 298 K. The formation of NH4+ monomer (band at 1410 cm−1) and N2H7+ dimer (1460 cm−1) was revealed by IR spectroscopy. Enthalpy of ammonia sorption on Dawson structure was calculated to be −127.9 kJ mol−1, indicating the lower acid strength of Dawson-type compared to that of the Keggin-type heteropolyacids, like H4SiW12O40.