The objectives of this study were: to assess changes in the organic matter (OM) fractions of the surface layer (0-20 cm) of the forested Ethiopian and Nigerian soils following their conversion to arable lands, to determine possible changes in the structural stability of these forested soils as they were converted to arable farmland and to evaluate which OM fractions influenced structural stability of these soils most. There were progressive reductions in all OM pools and aggregate stability as the forested soils were cultivated (which were reflected in the soil structure degradation following deforestation); however, the magnitude of reduction was site-specific. The surface soils at Sirinka (Ethiopia) and Umudike (Nigeria) had the highest cultivation-induced reductions in OM (71.7-76.2%), total carbohydrates (R-CHO) (65- 76.4%) and humic acids (83.4-95.5%). The humic acids (HA), fulvic acids (FA) and humin (HM) showed a fairly high correlation with MWD of the cultivated soils but the least with forested soils. With microaggregation indices, higher and more significant correlations were obtained. In the forested soils the WSA < 0.25 mm correlated negatively with HA (r = -0.886***), FA (r = -0.607*), HM (r = -0.772**), and with R-CHO (r = -0.687*). Also the CDR correlated negatively with HA (r = -0.828***), FA (r = -0.640*), HM (r = -0.716**) and with R-CHO (r = -0.626*) in the forested soils. Correlation of these OM fractions with these two micro- structural stability indices were slightly higher in the cultivated than forested soils and substantiate earlier results from the temperate regions in which these OM fractions were more effective in the enhancing of aggregate stability at the micro- than macro- aggregation levels.