A new Knudsen effusion apparatus based on the mass-loss technique was developed. The apparatus was designed for the measurement of vapour pressures below 1 Pa, at temperatures between 250 K and 470 K, using simultaneously up to seven effusion cells. The quality of the results obtained with this new apparatus was verified through the study of two reference compounds, anthracene and phenanthrene. Both the measured vapour pressures and the derived standard molar enthalpies of sublimation are in excellent agreement with values available in the literature for these compounds. (C) 2011 Elsevier Ltd. All rights reserved.