Changes in the chemical, viscoelastic and hygroscopic properties of wood cell walls in Chinese fir (Cunninghamia lanceolata) during the transition from sapwood to heartwood were studied to provide insights into the formation of heartwood. In situ imaging FTIR measurements indicated that the composition of the main components of cell walls remained almost unaltered, but more extractives were deposited in the wood cell walls during the sapwood-heartwood transition. Compared to the sapwood and transition wood, the heartwood had a higher softening temperature and greater activation energy, suggesting that the mobility restrictions of cell wall biopolymers were due to extractives obstructing the accessing of the plasticizer (ethylene glycol). The moisture sorption was the same from the sapwood to heartwood at a low relative humidity (RH), while the heartwood adsorbed less water at a high RH, probably caused by the extractives deposited in the matrix and mesopores of heartwood cell walls.