The microstructure of the ternary system pentaethyleneglycol monododecyl ether (C12E5), decane and water was investigated by static light scattering in the water-rich region of the phase diagram at constant temperature (22 degrees C). Micellar size and shape were determined along several pseudobinary sections of constant oil-to-surfactant ratio. The main focus was on dilute solutions (c(m) ranging from 0.0004 g cm(-3) up to 0.3 g cm(-3), where c(m) denotes the mass concentration of oil plus surfactant in the aqueous phase) and small oil content (alpha up to 0.35, where alpha denotes the mass fraction of oil in the oil + surfactant mixture). Our data analysis supports the presumed existence of wormlike micelles at low a. The concentration dependence of the apparent molar mass M-app of the micelles can be represented over a wide concentration range (up to the semidilute regime) by a model that combines a power law M-w proportional to c(m)(a) with the structure factor S(q = 0) for flexible polymer chains in good solvents. For alpha less than or equal to 0.10 the fitted value of the growth exponent a agrees with the mean-field prediction (a approximate to 0.5). With increasing oil content at given overall solute concentration c(m) the molar mass M-w of the micelles increases up to a maximum near a = 0.07 but decreases rapidly as the oil content is further increased. This decrease of M-w is due to a transition from elongated (wormlike) micelles to microemulsion droplets. At high oil content (a = 0.35) the data can be represented by a model of spherical droplets interacting like hard spheres.