To investigate the effects of phenolic extractives on the discoloration of black locust wood (Robinia pseudoacacia) during heat treatment, phenolic compounds were extracted using an accelerated solvent extraction. The main components of the phenolic extractives were analyzed. The phenolic compounds were heat treated at 120 and 140 °C in nitrogen, oxygen, and saturated steam. The results showed that the a* values shifted toward red and the b* values shifted toward yellow after the heat treatment. The changes in the color parameters were more pronounced when the samples were treated at 140 °C in saturated steam compared with treatment at 120 °C in oxygen or nitrogen atmosphere. During heat treatment, hydroxyl groups in the phenolic components were oxidized to form carbonyl groups, or the adjacent hydroxyl groups formed quinoid structures. It was possible that the sample underwent condensation reactions to produce conjugated double-bond structures that led to the increase in color parameters.