A modified ebulliometer and an apparatus based on comparative ebulliometry have been assembled for vapour-pressure measurements on high-boiling substances with melting temperatures as high as about 400K. The vapour pressures of 2-chlorobenzonitrile and 4-chlorobenzonitrile in the approximate temperature range : 380 K through 490 K, have been determined with estimated accuracy better than 0.04 K in the temperature and 0.1 per cent of the measured pressure value. The results thus obtained were correlated by the Antoine and Cragoe-Marks equations. Experimentally determined freezing temperatures of the two chlorobenzonitrile isomers and the calculated relative volatility of their binary mixture as a function of temperature are also reported.