An account is given of the manner in which plots, against mole fraction, of thc partial excess molar properties of water in amphiphile + water systems provide a sensitive means of identifying composition ranges over which significant variations take place in the prevalent modes of molecular aggregation. Examples are provided of instances where one may explore the effects of varying the alkyl chain lengths and polar head group sizes in the aqueous mixtures of alkyl poly(ethylene glycol) monoethers.