The proton affinity of N2O has been calculated with the combination of theoretical methods called Gaussian-1 theory (567·7 kJ mol-1) and Gaussian-2 theory (570·4 kJ mol-1). The site of protonation at the highest levels of theory (G1 and G2 theory) is predicted to be oxygen; however, this prediction is very sensitive to the level of electron correlation. At MP3, MP4SDQ, and QCISD(T) levels of electron correlation, O protonation is preferred, whereas at MP2 and MP4SDTQ levels, N protonation is preferred. At the MP4 level, the effect of triple excitations on relative energies is overestimated and strongly favors the HNNO+isomer over the HONN+isomer. At the QCISD level, the effect of triple excitations also favors the HNNO+isomer, but it is not sufficiently large to cause the N protonated isomer to become more stable. Evidence is presented indicating that biradical character in HNNO+leads to a nonlinear NNO bond angle.