A new method is presented that combines dual fluorescence microscopy and single-particle tracking (SPT) to investigate the impact of spatial inhomogeneities in membranes on lateral diffusion within Langmuir monolayers. In contrast to previous SPT experiments, random walks of individual fluorescent tracer particles and the structure of the membrane are simultaneously imaged. Monolayers of 1,2-dimyristoyl-sn-glycero-2-phosphoethanolamine (DMPE) in the liquid condensed/liquid expanded phase are used to model the inhomogeneous plasma membrane, while the fluorescent particles embedded in the membrane serve as model proteins. The interactions between tracers and the domain boundary and the resulting change in their random walks are directly observed. Since this technique permits systematic SPT studies of diffusion in inhomogeneous monolayers, its extensions to more biologically relevant monolayers are laid out as well.