Finally, we examine the behavior of the mixed monolayer upon exposure to 375 nm light. First, the monolayer composed of DPPC/E-CyP-OBu 9:1 was compressed to 20 mN/m at 25°C and the area was kept constant for the duration of the irradiation experiment. The compressed monolayer was either sequentially irradiated at 375 nm at different time or kept in the dark (Figure ). While the surface pressure shows an initial decrease from 20 to 18.7 mN/m after the Teflon barriers were stopped in the darker (confirming fluidification), it increases slightly during the first irradiation to 18.9 mN/m, due to the formation of the Z-isomer, which compresses the lipid monolayer, most probably due to its initial structural reorganization. After the darkening, the pressure decreases back to 18.5 mN/m, and the second irradiation, contrary to the first one, brings the surface pressure to 17.9 mN/m. This may be caused either by a more pronounced expulsion of the Z-isomer from the monolayer or by an even better, and more space-saving, arrangement at the surface. However, and interestingly, the decrease of the surface pressure is larger under irradiation than in the dark.