Ass ociation-dissociation equilibria and the static scattering function were formulated using precise thermodynamic functions for nonionic surfactant solutions including long, stiff, threadlike micelles. The present theory is applicable for micellar solutions with the surfactant concentration much higher than the critical micelle concentration and containing highly growing threadlike micelles. The scattering function formulated was compared with experimental light scattering data for aqueous solutions of a nonionic surfactant, penta(oxyethylene glycol) n-decyl ether (C12E5), at different surfactant concentrations and also temperatures.