Abstract: A method of extracting of thermoset epoxy fractions from a composite structure comprising thermoset epoxy matrix having a network structure and embedded solid element(s) is described. The method comprises providing a swelling fluid and soaking the composite structure with the swelling fluid including allowing the thermoset epoxy matrix to swell the swelling fluid into the network structure of the thermoset epoxy matrix for a sufficient time to mechanically break up the network structure to form a multitude of thermoset epoxy fractions liberated from the solid elements. Separating at least a portion of the thermoset epoxy fractions from the solid elements to obtain a swelling fluid slurry comprising the portion of the thermoset epoxy fractions in the swelling fluid. The swelling fluid comprises at least 1 mol/L of formic acid.