Simplified expressions have been developed for the engineering emissivity of uniformly distributed diatomic gases for nonoverlapping rotational lines with a resonance contour. Unfortunately the rotational half‐widths for spectral lines arising from transitions between excited vibrational energy levels are generally not known. For this reason it was necessary to make the assumption that the rotational half‐widths for transitions of the formn→n+v,v=1 or 2, are identical. The theoretical analysis is, however, sufficiently general to be useful without modification when accurate data concerning the dependence of line‐width on vibrational excitation become available. Explicit expressions have been obtained for the contributions to the total emissivity associated with individual vibration‐rotation bands.Representative emissivity calculations have been carried out for CO and HCl. Comparison of the calculated emissivities of CO with experimental data shows only fair agreement, suggesting either that the assumed description of rotational half‐widths is inadequate or else that the empirical emissivity data are not reliable at elevated temperatures.