The diameter of water-in-oil reverse micelles formed by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) with water and with protein Bovine serum alumin (BSA) was investigated by dynamic light scattering (DLS) measurement of AOT/isooctane solution at 298.15 K. The diameter and viscosity of AOT reverse micelle increases with increasing omega(0) (mole ratio of water to surfactant: omega(0) = n(w)/n(s),). The viscosity of AOT/isooctane with water or BSA inside did not change with shear rate, exhibiting Newtonian behavior. For omega(0) = 8.3, the diameter of AOT reverse micelles increases with increasing BSA concentration from 2 mg of BSA.mL(-1) of H2O to 145 mg of BSA.mL(-1) of H2O. The diameter of AOT reverse micelles increases only slightly with BSA inside compared with water inside at the same omega(0), Incorporation of protein into the micelles does not affect their mean diameter significantly. The hydrodynamic diameter of BSA from 10 mg to 100 mg of BSA.mL(-1) of H2O in 0.1 mol.L-1 AOT reverse micelles increased almost linearly with omega(0) which confirmed that the diffusion coefficient was independent of omega(0), indicating a spherical structure for the ACT reverse micelles. The localization of BSA is in the center part of the water pool without perturbation of the reverse micelles. For highly concentrated BSA from 50 ring to 145 mg.mL(-1) of water, only when omega(0) is higher than 5.6, the 0.1 mol.L-1 ACT reverse micelles can solubilize it.