Calorimetric measurements have been used to study the shape transitions of micelles. Partial molar enthalpies of solutions of cosurfactants (n-pentanol, benzyl alcohol, 3-methyl-l-butanol, n-hexylamine) have been measured in 2% TTMAB solutions at 25 and 45°C with various electrolytes (NaCl, NaBr, NaSCN, NaF, NaNO3, KCI, KBr, NaOH, LiCl, BaCl2, NaI) and nonelectrolytes (dioxane, urea, nhexylamine, benzyl alcohol, hexanol ). Some measurements were made on 3.5% SDDS solution at 45°C. The break in the plot of partial molar enthalpy of solution of cosurfactants vs cosurfactant concentration was observed in both TTMAB and SDDS solutions. The end of this break observed calorimetrically corresponds to the sphere-rod transition observed with light-scattering. Counterion ability to induce the TTMAB sphere-rod transition decreases in the order SCN- > NO3- > Br- > OH- > Cl- > F-. Co-ions were found to have a little or no effect on the concentration of pentanol at the end of the transition for a particular salt concentration. The effectiveness of nonelectrolytes in promoting the sphere-rod transition of 2% TTMAB decreases in the order n-hexanol >n-pentanol ∼ benzyl alcohol > 3-methyl1-butanol. n-Hexylamine was found to be less effective than n-pentanol in TTMAB as opposed to previous observations in SDDS. Urea and dioxane were found to retard the sphere-rod transition in TTMAB as in SDDS. Extrapolation of transition concentrations of 2% TTMAB and 3.5% SDDS containing cosurfactants (n-pentanol, n-hexanol, n-hexylamine) and salts to zero concentration of cosurfactants gives the threshold salt concentrations. Rodlike micelles are formed above the threshold salt concentrations, which are 0.171 ± 0.005 M NaBr at 25°C and 0.286 ± 0.010 M NaBr at 45°C in 2% TTMAB, and 0.438 ± 0.009 M NaCl at 25°C and 0.582 ± 0.014 M NaCl at 45°C in 3.5% SDDS. © 1992.