Abstract: The quartic force fields of FNO and CINO have been computed at the CCSD(T)/cc-pVTZ level. Using an ''augmented'' basis set dramatically improves results for FNO but has no significant effect for CINO. The best computed force field for FNO yields harmonic frequencies and fundamentals in excellent agreement with experiment. Overall, the force fields proposed in the present work are probably the most reliable ones ever published for these molecules. Total atomization energies have been computed using basis sets of spdfg quality: our best estimates are Sigma D-0 = 208.5 +/- 1 and 185.4 +/- 1 kcal/mol for FN0 and CINO, respectively. The computed value for FNO suggests a problem with the established experimental heat of formation. Thermodynamic tables in JANAF style at 100-2000 K are presented for both FNO and CINO.