The effects of varied concentration of 1-pentanol and 2-pentanol on the phase transitions of dipalmitoylphosphatidycholine (DPPC) bilayers were studied using differential scanning calorimetry (DSC). The threshold concentrations for 1-pentanol and 2-pentanol to induce the interdigitated phase were observed to be at about 0.06mM and 0.1mM respectively. These critical concentrations were indicated by the disappearance of the pre-transition and an increasing hysteresis between the main transition temperatures of the heating and cooling scans. At concentrations lower than the threshold, the main transition temperature decreased with increasing concentration for both 1- and 2-pentanol; however, the behavior of the main transition temperature in 1- and 2-pentanol differed slightly at concentrations higher than the threshold. Neither of the isomers demonstrated the biphasic effect, but did show a change in slope of the main transition temperature. At concentrations higher than the threshold, the main transition temperatures of DPPC in 1-pentanol did not increase with increasing concentration, while that of DPPC in 2-pentanol clearly increased but in a small concentration range. Both isomers showed a decrease in the main transition temperature at higher pentanol concentrations. The observed difference in the threshold concentrations implied that branching decreased its effectiveness in inducing the interdigitated phase in DPPC.