We present an analysis of the complex of water with CO which includes (a) a new potential energy surface obtained by fitting ab initio points, followed by adjustment against experimental rotational, spin-spin and quadrupole coupling constants; (b) diffusion Monte Carlo (DMC) studies of the ground vibrational state, and of three excited vibrational states, at J = 0. A new approach is suggested for DMC calculation of intermolecular vibrational frequencies.