Polymer-tethered phospholipid bilayers, which are based on a phospholipid-lipopolymer mixture, represent a very promising approach to stabilize complex biomimicking composite membranes. Furthermore, they are interesting model systems to study problems of hindered diffusion in two-dimensional liquids. Here, we present fluorescence recovery after photobleaching experiments (FRAP) on mixed phospholipid-lipopolymer monolayers of DMPC and DSPE-EO45 at the air-water interface. In contrast to recent polymer-tethered bilayer experiments where the hydrophobic lipopolymer anchors behaved as immobile obstacles within the fluid phospholipid matrix, this paper investigates the influence of mobile lipopolymer obstacles on the lateral diffusion of phospholipids. We found that the lateral diffusion of phospholipids with D = 7.1 +/- 0.5 mum(2)/s is independent of the lipopolymer obstacle concentration if adjacent polymer chains do not interact with each other. However, the diffusion coefficient of nontethered phospholipids gradually decreases from D = 7.1 +/- 0.5 mum(2)/s to D = 3.4 +/- 0.1 mum(2)/s in the case of increasing polymer-polymer interactions based on frictional coupling. This can be understood by a slowing down of the obstacle mobility. While phospholipids still show a significant lateral diffusion as long as the polymer moieties interact with each other only via frictional coupling, they become rather immobile (D = 0.9 +/- 0.1 mum(2)/s) if lipopolymers form a two-dimensional physical network.