This research aims at studying experimentally the surface processes occurring during burning of a solid polymeric fuel containing a small fraction of expandable graphite (EG) particles/flakes (nominal size 150 mu m). Solid polymeric fuels burning in oxidizer or air streams are found in propulsion devices such as hybrid rocket motors or solid fuel ramjets. It was revealed that the inclusion of EG particles in the fuel may increase its burning (regression) rate. Expandable graphite is a specific type of intercalated graphite, which, at elevated temperatures, tends to swell and form worm-like strings several folds longer than the original particles. The behavior of these strings and the dynamic phenomena at the fuel surface subjected to a flame at atmospheric conditions have been visualized using high-speed (1000 pictures per second) video photography. Formation and growth of the EG strings have been observed revealing characteristic times of tens of milliseconds and a typical length in a millimeter range. Individual EG strings emit from the surface and are eventually entrained by the gas flow without causing crumbling or detachment of layers from the fuel surface.