At Mt. Hirugatake in the Tanzawa Mountains, Kanto district, Japan, the deciduous broadleaved forests have rapidly declined. In our previous studies, we reported that the amount of soil organic matter had significantly decreased at the early and final stages of forest decline, and that the soil microbial biomass also showed a large decrease at these stages, suggesting that the composition of soil organic matter might have also changed with forest decline. To clarify the influences of forest decline on the composition of soil organic matter, the amount of humic substances, optical properties of humic acid, and the amount of soil carbohydrates in surface soils at different stages of forest decline were investigated. The amounts of humic acid and fulvic acid decreased to a lesser extent at the early and middle stages of forest decline, and showed a significant decrease at the final stage. As the amount of humin significantly decreased at the early stage, it was plausible that the distinct decrease in the total carbon content of the soil surface horizons at the early stage of forest decline was induced by the decrease in the amount of humin, and at the final stage, by the decrease in the amounts of humic acid and fulvic acid. The amount of soil carbohydrates did not change appreciably with forest decline although the soil organic matter content markedly decreased. It was suggested that most of the carbohydrates in the soil surface horizons were in a stabilized form consisting of complexes with humic substances, metals, and minerals, and would not be affected by the environmental changes associated with forest decline.