The Ostwald's partition coefficient for the solubility of methyl iodide in water was determined from 4.85 to 68.5 °C. The partition coefficient measured for 10−7to 10−8 Msolutions was found to be from 20 to 30% lower than for 10−2 Msolutions. A new rapid gas chromatographic technique is described for the determination of partition coefficients in dilute systems.