Proutière's theory of molecular weight (M) determination by Rayleigh scattering has been tested on surfactants in solution, and it has been compared to the classical Carr-Zimm (CZ) method. Both methods make use of time-averaged scattering intensity, refractive index, and density measurements. Monoethers of poly(ethylene glycol) samples have been studied in the range 0.33-4.67 kg/mol of M. Under those conditions, it has been shown that Proutière's method leads to a far better agreement with formula weight (M) values than the CZ method because Proutière theory takes into account the specific mean polarizability of the solvent and derivative of the solution density with respect to the solute mass concentration, which is counter to CZ theory.