We report the application of the transient grating method to measurement of radii of inverse micelles. A theoretical model is used to calculate the time evolution of the grating as a function of the solution thermal properties and the radii of the micelles. The method is based on the large difference in the thermal expansion coefficients between water and organic solvents : energy deposited in the aqueous interior of the micelles by the optical exciting beams does not result in sizeable photoacoustic effect until heat diffuses into the surrounding organic solvent. The time profile of the transient grating signal is shown to be a function of the micellar radius, and the thermal conductivity and thermal diffusivity of the aqueous and organic subphases. For water-in-alkane reverse micelles, heat deposition is limited to the aqueous interior of the micelles through the use of ionic dyes. Experiments are reported using 1-3 GHz acoustic waves in solutions containing micelles with diameters in the range 10-30 nm. The correspondence between radii determined by the thermal diffusion method and those from dynamic light scattering measurements is discussed.