Recent interest in using submicrometer particles for industrial and therapeutic purposes has led to concerns about their interactions with biological membranes. The mechanisms of particle-membrane interactions are not well understood resulting in contradictory reports on the effects of particles on membrane interfacial properties. In this study, the interactions between negatively charged polystyrene particles (200 nm) and monolayers of dipalmitoyl phosphatidylcholine (DPPC) were investigated. Surface pressure, surface potential, and surfactant microstructure studies were conducted to monitor the interfacial properties of DPPC monolayers spread on a subphase in which particles were dispersed. At a concentration of 0.1 g/L, particles caused a partial collapse of the monolayer. DPPC monolayers spread on a particle-laden subphase also exhibited higher surface potential and increased ratio of ordered domains supporting the presence of a more compact monolayer. These results suggest that particles penetrated the air-water interface thereby altering monolayer packing at the interface. These findings are contrary to our previous work where particles injected into the subphase beneath a DPPC monolayer did not penetrate the interface confirming that the sequence of particle and monolayer addition can alter particle-monolayer interactions. These studies may partially explain the varying results reported in previous studies.