Differential scanning calorimetry (DSC) and fluorescence spectroscopy were used to investigate the phase behavior of hydrated dipalmitoylphosphatidylcholine (DPPC), a 16 carbon chain phospholipid, upon mixing with a self interdigitating phospholipid (SIP). Interdigitation is a special phase of bilayer membranes in which the phospholipids acyl chains interpenetrate, halving membrane thickness. Mixtures of DPPC and SIP showed increasing main transition temperatures (Tm?s) with rising SIP percentage. Consecutive heating and cooling scans had shifting Tm?s, shoulder peaks appeared or disappeared and an increasing trend in hysteresis between heating and cooling Tm was discovered in presence of 30-50% SIP. For the fluorescence studies, 2-(3-(diphenylhexatrienyl)propanoyl)-1-hexadecanoyl-sn-glycero-3-phosphocholine (β-DPH HPC), a phospholipid bound probe, was added in low concentration to the DPPC - SIP mixtures to determine the percentage SIP which causes bilayers to interdigitate. Decreasing fluorescence intensity is directly correlated to exposure of β-DPH HPC to aqueous environment, as occurs with interdigitation. This appeared to be at 30% SIP. The trend in hysteresis and decrease in fluorescence indicate that a ~40% ratio of SIP : DPPC can induce interdigitation of bilayer membranes.