Dynamic and static photophysical and spectroscopic measurements have been used to analyze the microdomains of interpenetrating network (IPN) films consisting of 1:1 polyethylene:(7:3)-poly(butyl methacrylate)-co-polystyrene. The primary probe, fluorescence from pyrenyl groups covalently attached selectively to the different elements of the IPN, has been monitored under a variety of conditions, including in the absence and presence of a fluorescence quencher, diethyl oxalate (DEO), which is not able to enter domains of polyethylene. It has been possible to discern the parts of the IPN that are accessible to molecules of DEO and the rates at which DEO is able to enter them. Two diffusion coefficients, indicating parallel diffusion pathways, are necessary to account for DEO entry into several of the films. The IPN consists of largely segregated domains, but there are extensive interactions between domains at their interfaces. Conclusions are derived from comparisons of results from the IPN and films of their constituent polymeric elements when each is tagged with the lumophoric groups, as well as from films containing noncovalently linked pyrenyl groups. These results help to clarify the picture of the network derived from structural studies by providing dynamic insights into degrees of domain interpenetration and permeability.