High-vacuum naphthalene evaporation loss experiments were conducted with a specially designed annular effusion cell to examine the validity of the most frequently used theoretical models of molecular flow, specifically applied to a high molecular weight organic compound. Literature data are similarly correlated to (a) show the respective accuracy of the various formulae and (b) discuss the most prevalent effusion experimental errors often causing orders of magnitude differences between theoretical predictions and experimental results. A simplified molecular flow effusion model is proposed, modified to account for the evaporation rate and rate of vapor pressure change of polymeric lubricating oils. The results and conclusions are useful for accurate predictions of long-term lubricant loss through annular-design labyrinth seals of wet-lubricated aerospace mechanisms and vacuum chamber components.Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference held in Atlanta, Georgia, October 16–18, 1973