Wormlike micelles of the surfactant penta-, hexa-, and heptaoxyethylene dodecyl ethers C12E5, C12E6, and C12E7 were characterized by static light scattering (SLS) and dynamic light scattering (DLS) experiments to examine effects of uptake of n-dodecane on the micellar characteristics. The SLS results have been successfully analyzed by the light scattering theory for micelle solutions to yield the molar mass M-w(c) as a function of concentration c along with the cross-sectional diameter d of the micelle. The apparent hydrodynamic radius R-H,R-app(c) determined by DLS as a function of c has also been successfully analyzed by the fuzzy cylinder theory which takes into account the hydrodynamic and direct collision interactions among micelles, providing us with the values of the stiffness parameter lambda(-1). It has been found that the micellar length L-w increases with increasing surfactant mass concentration c and the values of d and lambda(-1) increase with increasing n-dodecane content W-d, as in the case of various CiEj micelles containing n-alcohol. On the other hand, the values of M-w L-w and R-H,R-app for all the micelles examined decrease with increasing Wd contrary to the micelles containing n-alcohol. This finding may be attributed to the fact that the addition of n-dodecane into the micelles weakens hydrophilic interactions among polyoxyethylene chains of the surfactant molecules and water, making the micelles unstable, and then leading them to collapse into smaller micelles.