The dehydration effects of varying concentrations of ammonium sulfate on phospholipid dipalmitoylphosphatidylcholine (DPPC) bilayer phases were studied using differential scanning calorimetry (DSC) and fluorescence spectroscopy. 1,6-diphenyl-1,3,5-hexatriene (DPH) was used as a fluorescence probe to monitor phase transitions. Changes in acyl packing and hydration within the bilayer during phase transitions force the hydrophobic DPH molecule into and out of more hydrated regions, changing the fluorescence intensity of DPH. A decrease in fluorescence intensity occurred during subgel formation due to dehydration forcing DPH into a more hydrated region. Increases in both the pre and main transition temperatures with increasing concentrations of ammonium sulfate suggest that ammonium sulfate dehydrates the liquid crystalline phase in favor of one of the two gel phases. Also, a smaller decrease in fluorescence intensity during subgel formation and a decrease in the subtransition temperature with increasing ammonium sulfate concentrations suggest that ammonium sulfate causes the formation of a less stable, more hydrated subgel. Finally, multiple endothermic peaks for the subgel shown through DSC indicate that the kinetics of subgel formation for DPPC includes several steps due to domain growth. The formation of a homogenous, fully dehydrated DPPC subgel may require longer incubation times than other phospholipid systems.