A new Knudsen effusion apparatus, enabling the simultaneous operation of nine effusion cells at three different temperatures, is fully described. The performance of the new apparatus was checked by measuring the vapour pressures, between 0.1Pa and 1Pa, over ca. 20K temperature intervals of benzoic acid, phenanthrene, anthracene, benzanthrone, and 1,3,5-triphenylbenzene. The derived standard molar enthalpies of sublimation are in excellent agreement with the mean of the literature values available for these five compounds and with the recommended values for four of them.