We studied the viscosity behavior of 500 cSt dimethyl silicone oil with considerable concentration of absorbed air. For this purpose, a Couette rheometer with a transparent rotating outer cylinder and a fixed inner cylinder was established, in which the ambient pressure is controlled by a piston-cylinder apparatus. Careful torque calibration and equilibrium examination for the air-silicone-oil mixture were carried out. In the range of ambient pressure from 1 to 5 atm, corresponding to the air molar fraction from 11 to 35%, it was found that viscosity of the silicone oil decreases monotonically as the air concentration increases. The viscosity variation can be explained by the Eyring's model in which the free energy of activation is modified to account for the nonideality of the air-silicone-oil mixture. The excess free energy used to fit the viscosity variation is approximately equal to that used in the Krichevsky-Ilinskaya equation, which accounts for the nonideal solubility of the air-silicone-oil solution. The activation energy in the Arrhenius equation for viscosity-temperature relationship is identified as the activation enthalpy. (C) 2011 American Institute of Chemical Engineers AIChE J, 57: 3299-3304, 2011