Abstract. A design of and initial results from a Knudsen Effusion Mass Spectrometer (KEMS) are presented. The design was adapted from high temperature alloy studies with a view to using it to measure vapour pressures for low volatility organics. The system uses a temperature controlled cell with an effusive orifice. This produces a molecular beam which is sampled by a quadropole mass spectrometer with electron impact ionization calibrated to a known vapour pressure. We have determined P_{(298 K)} and ΔH_sub of the first 5 saturated straight chain dicarboxylic acids: 2.15±1.19×10^-2 Pa and 75±19 KJ mol⁻¹ respectively for oxalic acid, 5.73±1.14×10^-4 Pa and 91±4 KJ mol⁻¹ for Malonic acid, 1.13±0.47×10^-4 Pa and 93±6 KJ mol⁻¹ for Succinic acid, 4.21±1.66×10^-4 Pa and 123±22 KJ mol⁻¹ for Glutaric acid and 6.09±3.85×10^-6 Pa and 125±40 KJ mol⁻¹ for Adipic acid.