Incremental calorimetric titration of pentanol into aqueous solutions containing 3.5% cationic (tetradecyltrimethylammonium bromide, TTMAB, in 0.05 M NaBr) or anionic (sodium dodecyl sulfate, SDS, in 0.28M NaCl) surfactant and solubilized additives induced the transformation of spherical to rodlike micelles. The effect of the additive on the transition has been quantified as the change in the concentration of pentanol required to cause the transition divided by the concentration of additive, with a negative value indicating a cooperative effect of promoting the transition and a positive value indicating a retarding effect. Aliphatic hydrocarbon additives retarded the transformation for both surfactants, with the exception of a small cooperative effect for cyclohexane on TTMAB. Alkylbenzenes promoted the transition for TTMAB, as did benzene, toluene, and ethylbenzene for SDS, but the propyl and butyl derivatives showed the opposite effect. © 1993 American Chemical Society.