In relation to the problem of global warming, forest ecosystems are expected to be possible sinks of atmospheric CO₂. This idea includes the role of forest soils as a large pool of organic matter. Based on field and laboratory measurements on Soil Organic Matter (SOM) dynamics in different regions of humid Asia (Japan, Thailand and Indonesia and some Eurasian steppe soils for comparison) with different soil pH levels, we postulate that the soil acidity play an important role to accumulate SOM in the forest soils through following different processes. (1) Incubation experiment shows that strong acidity largely decreases the decomposition rate constant (k) of SOM. The values of k from strongly acidic soils below pH 5 (mainly Udepts and Udults from Japan and Kalimantan of Indonesia) were much lower than those from Eurasian steppe soils (mainly Mollisols; nearly neutral in pH). As a result, the amount of potentially mineralized C (C₀) of the Kalimantan soils or light fraction-C in the Japanese soils drastically increased. Thus the strong soil acidity can enhance the accumulation of mineralizable organic materials in the surface soils through decreasing soil microbial activities for SOM decomposition. (2) According to the field measurements on C flux in different forest soil ecosystems, the internal leaching of Dissolved Organic Carbon (DOC) from the litter layers or surface soils increased under low pH conditions around 4, presumably due to predominated fungal activity. Most of DOC thus mobilized was accumulated again in the subsoils. Such a DOC flux sometimes amounted > 10% of annual CO₂ emission from the soil surface. It was typically observed in acidic soils under udic soil moisture regime, i.e. Udepts in Japan and Udults in Kalimantan, indicating that the downward movement of DOC in acid soils increases the chance of subsoils to accumulate SOM as organo-mineral complexes especially in north temperate forest. The intensity of soil acidity generally reflects climatic conditions and partially parent materials of soils. In Japan, non-volcanic soils exhibited lower pH values and hence a larger internal DOC flux than volcanic-derived soils. In tropical regions, strongly acidic soils (Udults) in Kalimantan exhibited lower k and higher C₀ values during SOM decomposition as well as higher DOC flux in the soil profile compared to Ustults in northern Thailand. When considering C dynamics in forest soils, we must take account of the effect of soil acidity on SOM decomposition more carefully, which might have been negligible in steppe or cropland ecosystems.