A thorough investigation of the acid-catalyzed polycondensation of furfuryl alcohol was conducted with the specific aim of understanding the mechanisms responsible for the main reaction, but more particularly for the transformation of linear unconjugated oligomers into black cross-linked final materials. The originality of the present approach, compared with previous unsuccessful attempts, consisted in calling upon numerous model compounds simulating specific features of both the monomer and its condensation products. The formation of conjugated sequences along the poly(furfuryl) chains is caused by repetitive cycles involving the loss of hydride ions followed by the deprotonation of the carbenium ions thus formed. This reaction was simulated and accelerated by using cationic hydride-ion abstractors. The branching reactions only occur after the appearance of the multiple unsaturations and owe their origin to interchain cycloadditions between furan rings and conjugated structures. A long-standing puzzle has thus been solved.