Static and dynamic light scattering and surface tension measurements were used to study the effects of various pentanol isomers on sodium dodecyl sulfate (SDS) micelles in 0.5 M NaCl. The addition of any of the pentanol isomers increased both the average hydrodynamic radius, [R-h], and the average radius of gyration, [R-g]; however, the magnitude of the effect depended on both temperature and pentanol isomer. By comparing the ratio of these two measures of micellar size, it was found that the addition of pentanol promotes an elongation of the rodlike micelles. The effects of the pentanol isomers on the critical micellar concentration also were determined. Once more the various isomers did not all behave identically. Last, the effect of temperature on micellar size in the presence of the various isomers was determined. In all cases an increase in temperature resulted in a decrease in mean micellar size; once more the overall effect depended on the isomer in solution.