The vapor pressures of primary coal tars have been determined using a modified Knudsen effusion method. Vapor pressure data are correlated with the molecular weight and temperature for tars from three Argonne coal samples (Pittsburgh No. 8, Upper Freeport, and Wyodak). These results show significant differences in the vapor pressures of similar molecular-weight tars from different coals. The results likewise warn that the commonly used estimation techniques for vapor pressures of the coal tars oversimplify the actual behavior but at the same time provide reasonable estimates for some modeling applications.