In this study, a rapid and versatile methodology for quantitative pore structure characterization is described taking the example of intumescent char. The char porosity and pore structure tend to be difficult to quantify but are decisive for the material’s thermal conductivity and thus performance. The methodology described here uses a fluorescent dye containing epoxy to impregnate the char. After curing, the sample is cut and polished, followed by imaging with a fluorescence microscope. The recorded images are segmented using an open-source interactive machine learning program, followed by automated pore size and porosity measurements. Volume-based three-dimensional (3D) pore size distributions are calculated via stereography and compared to X-ray microcomputed tomography (μ-CT) results. The pore aspect ratio and tortuosity of the solid char phase are also discussed. It is shown that the fluorescence microscopy methodology produces high-resolution images with excellent contrast, while similar pore metrics (porosity of 81.8%, average volume-based pore size of 70.5 μm) to those for μ-CT are obtained.