Blends of amphiphilic poly(dimethylsiloxane) (PDMS) and a model polyhedral oligomeric silsesquioxane (POSS) nanofiller, trisilanolisobutyl-POSS, form homogeneous monolayers at the air/water interface. At high POSS content (>80 wt %), a thermodynamic analysis of surface pressure-area per monomer (Pi-A) isotherm data suggests possible phase separation prior to collapse of the PDMS component. PDMS bilayer formation that normally occurs around Pi approximate to 8 mN m(-1) shifts to higher surface pressures with increasing trisilanolisobutyl-POSS content, indicating compatibility between the two materials in the monolayer state that is analogous to a positive deviation binary liquid mixture. Further compression leads to the collapse of the POSS component into multilayer domains around Pi approximate to 17.5 mN m(-1). Brewster angle microscopy shows the amount of PDMS in the blend alters the aggregation state of the nonequilibrium solidlike trisilanolisobutyl-POSS domains. These results provide insight into POSS aggregation mechanisms that may be present in thicker films and bulk systems.