Studies on the vaporization of pure zirconium tetrachloride and hafnium tetrachloride have been conducted in the temperature range 400-520 K by the transpiration technique, using high purity argon as the carrier gas. Chemical and X-ray diffraction analyses of the condensate and of the residue confirmed the vaporization to be congruent. Assuming the vaporising species to be monomeric, the vapour pressures were calculated, and are given by the following equations logp((Pa))=(13.51 +/- 0.058)-(5164.95 +/- 26.0)/T (405.3 < T < 518.1 K) for [ZrCl4] reversible arrow (ZrCl4) and logp((Pa))=(13.64 +/- 0.147)-(5112.27 +/- 63.4)/T (398.4 < T < 500.3 K) for [HfCl4] reversible arrow (HfCl4)