Self-assembly of nanostructures on surfaces is a promising area in the emerging field of "bottom-up nanolithography". We describe a systematic analysis of hydrophobically capped gold nanoparticle (Au NP) assemblies created within monolayers of saturated phospholipids deposited at the air/water interface. We show that the Au NPs are segregated within the mixed monolayers, forming distinct configurations. Microscopy analysis reveals that organized Au NP aggregates. including wires, rings, and "doughnut-shape" structures, are observed only within condensed-phase monolayers comprising phospholipids exhibiting longer acyl side-chains. In these monolayers, the Au NPs arc localized at the edges of the condensed phospholipid domains. In addition to the pronounced effect of the phospholipid phases at the air/water interface, NP organization was found to depend upon the hydrophobic capping agents of the particles. The Au nanostructures assembled at the air/water interlace can be transferred onto solid substrates, suggesting that the self-assembly monolayer approach could be exploited for practical nanoelectronic and sensing applications.