The time-dependent topology of domains in supported phospholipid bilayers of a binary mixture of dioleoylphosphatidylcholine and dipalmitoylphosphatidylcholine under a buffer solution has been studied by atomic force microscopy. We observe a transient regime of the phase separation until 45 min after a temperature quench from a miscible state of the system into the gel-liquid crystal coexistence region with the earliest observation after 20 min showing large gel-phase domains (containing similar to 10(4)-10(6) molecules) of irregular shapes. The transient regime is characterized by a power law for the growth rate of the domain size (A) with n = 3.0 +/- 0.4 in A proportional to t(2/n). After 45 min, an asymptotic power law with n = 20 +/- 10 is observed and is linked to an inhibited domain growth. The evolution of individual domains suggests that domain growth in the transient regime is governed by a ripening mechanism. The growth inhibition is linked to the observation that the gel domains in each leaflet of the bilayer must grow simultaneously at the same sites as they remain superimposed on each other throughout the phase separation process.