The structural dynamics of the main phase transition of large unilamellar dipalmitoylphosphocholine (DPPC) vesicles were studied using differential scanning calorimetry (DSC) as well as steady-state and time-resolved fluorescence spectroscopy. Two membrane-incorporated fluorescent lipid analogues, 1-palmitoyl-2-[10-(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine (PPDPC) and 1-palmitoyl-2-(3-(diphenylhexatrienyl) propanoyl)-sn-glycero-3-phosphocholine (DPHPC), were employed. In gel-state DPPC, the excimer formation times (tau(R)) for PPDPC (mole fraction X-PPDPC < 0.02) decrease with increasing temperature up to (T - T-m) &AP; -10 and together with high values for the excimer/monomer emission ratio (I-e/I-m) suggest an enrichment of this probe in the clusters. A pronounced prolongation of τ(R) observed with further increases in temperature and starting at (T - T-m) &AP; -10 is accompanied by only a modest decrement in I-e. This behavior is interpreted as with the dispersion of the clusters and the enrichment of PPDPC into the interfacial boundary separating emerging "fluid" domains from the bulk gel phase. Rapidly decreasing excimer intensity, fluorescence quantum yield, and acyl chain order within the temperature range of -4 < (T - T-m) < 0 together with a slight increment in τ(R) suggest the gradual disappearance of the phase boundary upon approaching T-m. Accordingly, our data indicate the possibility that there is an intermediate phase of a strongly fluctuating lattice of fluidlike ("excited") and gel-like ("ground") lipids, its formation starting at (T - T-m,) = -4 and being complete at T-m. With further increases in temperature (T - T-m > 0), this intermediate phase would transform into the liquid disordered phase as a second-order process with an increment in trans --> gauche isomerization corresponding to approximately one-third of the total transition enthalpy.