||An accurate quartic force field for formaldehyde is obtained by refining the ab initio quartic force field of Martin, Lee, and Taylor [J. Mol. Spectrosc. 160, 105 (1993)]. The refinement was achieved by iteratively fitting a subset of the coefficients of a Taylor-series expansion of the potential-energy surface to 138 of the observed transition frequencies, many of which were obtained by Bouwens et al. [J. Chem. Phys. 104, 460 (1994)] using dispersed fluorescence spectroscopy. We fit the vibrational energies (less than or equal to 7600 cm(-1)) for 138 states with an absolute mean deviation of 1.5 cm(-1). (C) 1996 American Institute of Physics.