The anharmonic force field of N2O has been determined from high-quality ab initio calculations. The results suggest that among the available experimental sextic force fields the field obtained by algebraic contact transformation is the most reliable, although even in this experimental field most higher-order terms (especially at fifth and sixth order) appear to be phenomenological in nature, i.e., do not directly correspond to the associated derivatives of the potential energy surface. The accuracy of computed force fields has further been demonstrated by anharmonic force field calculations on the diatomics (X) over tilde (1) Sigma(g)(+) N-2 and (X) over tilde (2) Pi NO.