Vapor pressure measurements have been made on pure iron pentacarbonyl between +31° and −19°C. The experimental results may be expressed in the form which corresponds to a heat of vaporization for the liquid carbonyl of . This result confirms and extends the earlier measurements made by Trautz and Badstiibner(2) between 0° and 140°C. The need for careful purification of commercially available iron pentacarbonyl is emphasized, particularly for establishing the correct vapor pressure below 45°C.