This study investigates the interactions between low- and high-molecular-weight portions of lignin during fast pyrolysis at 100-300 degrees C. Lignin was extracted via tetrahydrofuran (THF) into the THF-soluble and -insoluble fractions, which represent the low-molecular-weight (LMW) and high-molecular-weight (HMW) portions of lignin, respectively. As pyrolysis temperature increases, the char yield of the LMW lignin decreases more significantly than that of the HMW lignin. During fast pyrolysis, the experimentally measured char yields from the whole lignin are consistently higher than those calculated from the respective char yields of the LMW and HMW portions via addition, increasing from similar to 0.5 wt % at 100 degrees C to similar to 10.7 wt % at 300 degrees C. Such char yield differences provide direct evidence on the existence of interactions between the LMW and HMW portions of lignin during fast pyrolysis of the whole lignin. The results suggest that the formation of the liquid intermediate phase from the LMW portion suppresses the release of volatiles from both the LMW and HMW portions, resulting in a higher char yield during fast pyrolysis of the whole lignin. Experimental results further show that such interactions also result in changes in lignin char structures (especially at temperatures of >150 degrees C) because of enhanced polymerization reactions of oxygenated functional groups and aromatic structures with fused rings of 2-5. As a result, more condensed char structures are formed, as evidenced by the decreased atomic O/C and H/C ratios for the chars produced from the whole lignin pyrolysis.