The experiments described here were designed to determine the characteristic width delta of the cylindrical interface in a block copolymer that forms a hexagonal phase in the bulk state. We used fluorescence resonance energy transfer (FRET) to study films consisting of mixtures of donor- and acceptor-labeled poly-(isoprene-b-methyl methacrylate) (PI-PMMA, 29 vol % PI). Dye labels were attached at the PI-PMMA junctions. Because the dyes are connected to the junctions, they are confined to the block copolymer interface. To obtain information about the width of the interface separating the components, we compared measured and simulated donor fluorescence decay profiles. We developed a Monte Carlo method to introduce donor and acceptor sites within the interface and then used this dye distribution to calculate simulated "experimental" decay profiles. In this way we were able to examine the appropriateness of using a preaveraged value of the orientation parameter in evaluating FRET data. We found that a value of <|kappa|>(2) = 0.40 gave a reasonable agreement with results obtained by considering random orientation of dipoles for each donor-acceptor pair. Through these experiments, we found a value of delta slightly smaller than 1.0 nm, confirming that PI and PMMA are very meagerly miscible polymers.