New measurements of carbon monoxide laser frequencies in the 5.3 to 7.2 μm region have been made using the techniques of infrared grating spectroscopy and heterodyne beat frequency detection. The observations concentrated on the rarer isotopic species 13C16O, 12C18O, and 13C18O, and on the lower frequency region of CO laser oscillation, where less data have previously been available. The new results have been combined with a large amount of previous data in a simultaneous least-squares fit involving all known isotopic species of CO. The resulting parameters are a set of 'reduced' Dunham coefficients and a set of mass-scaling factors which describe how their isotopic variation differs from the simple mass-dependence normally assumed. Laser frequencies calculated from the new Dunham coefficients are improved from those previously available, especially in the relative accuracy of each isotope compared to the others, and they should prove useful in spectroscopic applications of the CO laser. Comprehensive tables of Dunham coefficients for all CO isotopes and of calculated laser frequencies for 12C16O,13C16O, 12C18O, and 13C18O are given in the paper and in an Appendix.
