Silicones are an important class of hydrophobic compounds in widespread use. To evaluate their fate in the environment, an accurate value of the air-water partition (Henry's law) constant is necessary, which, unfortunately, is lacking at present. A static head space and a newly developed dynamic vapor entry loop method were used to obtain the air-water partition constant for six volatile methyl siloxanes. Internally consistent data were obtained. The value of Henry's constant, as calculated from pure component vapor pressure and aqueous solubility, was 10- to 170-fold greater than the experimental values. A correction to the Henry's constant that involves the ratio of the thermodynamic activity coefficient for a methyl siloxane at infinite dilution to that at saturation solubility in the aqueous phase is proposed.