The molecular interaction of quercetin-3-O-palmitate (QP) with dimyristoylphosphatidylcholine (DMPC) has been studied. Film balance measurements of the average molecular area vs QP molar fraction in DMPC/QP mixed monolayers showed that relevant positive deviations from ideality, i.e., a less dense monolayer packing, occurred for a temperature of 10 degrees C, below the critical melting transition temperature of DMPC monolayers T c m approximately equal 20 degrees C), while ideal behavior was observed at 37 degrees C, above this phase transition temperature. The positive deviation observed at low temperatures in the average molecular area increased with the surface pressure. Scanning probe microscopy measurements performed on mixed monolayers transferred on mica showed that the deviations from ideality were connected to the formation of nanometric-scale QP-rich domains. However, the formation of aggregates was observed only for relatively high-QP molar fractions X QP > or = 0.25 at 10 degrees C, while it was not observed at 37 degrees C, i.e., when the ideal mixing was found at the air/water interface. The observed effects are explained in terms of a temperature- and surface pressure-dependent phase-separation process based on the predominance at low temperature and low molecular mobility of QP-QP and DMPC-DMPC aggregation forces, prompting the formation of QP-rich domains embedded in a DMPC-rich matrix. High temperature prompts the QP/DMPC ideal mixing.