Differential scanning calorimetry (DSC) was used to analyze the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) bilayers in the presence of pentanol isomers. Heating and cooling scans were run as a function of concentration of each isomer. Sufficient data were collected to determine the concentration of each pentanol needed to induce the interdigitated phase by the appearance of a biphasic effect in the main transition temperatures, the onset of a hysteresis associated with the main transition from gel-to-liquid crystalline phase, and the disappearance of the pretransition. Lower threshold concentrations were found to correlate with isomers of greater alkyl chain length while branching of the alkyl chain was found to increase biphasic behavior. The addition of a methyl group to butanol systems drastically decreased threshold concentrations. However, as demonstrated in the DPPC/neopentyl alcohol system, branching of the alkyl chain away from the ?OH group lowers the threshold concentration while maintaining a biphasic effect.