A thermodynamic model including fluctuations of micelle sizes has been derived to describe solution properties of amphiphile systems close to the critical micelle concentration. Owing to the consideration of an affinity field in the free energy of the system, the model is capable of featuring experimental findings that are incorrectly reflected by established theories of the micelle formation and disintegration kinetics. In conformity with experiments, the thermodynamic theory predicts the onset of micellar structure formation already at amphiphile concentrations below the critical micelle concentration. It also applies well for the distinctive concentration dependency of the relaxation rate of monomer exchange and likewise for the sound velocity and thus the isentropic compressibility variations with concentration. Comparison of the theoretical predictions is made with special emphasis to short-chain surfactant systems, which allowed for reliable measurements in the range below the critical micelle concentration.