Abstract: Organic matter (OM) is known to play a role in maintaining soil fertility through its influence on surface charge to adsorb nutrient ions in soils. Its effect is vital but variable. In this study, 80 soil samples were collected from 26 profiles of highly weathered Oxisols and Ultisols for the analyses of soil properties and surface charge characteristics. Oxisols have developed on residuum derived from basalt and limestone, whereas Ultisols have developed on residuum, alluvium, and colluviums derived from igneous, metamorphic, and sedimentary rocks. Charge behavior was determined on four sets of samples including whole soils, soils pretreated with NaOCl to remove OM, soils after sand fraction removal by ultrasonic probe, and soils after sand fraction removal and OM removal by NaOCl. Results of the study revealed that the soil’s charge behavior was dominated by variable charge for both Oxisols and Ultisols, and that the negative surface charge increased with increasing pH. Soils formed under humid conditions had a high positive surface charge attributed to the higher contents of goethite than hematite. Both negative and positive charges were high in topsoils of Oxisols. The negative surface charges decreased and the positive surface charges increased after removal of OM. Nevertheless, the surface charges in soil after removal of sand and of soil after removal of sand plus OM were not different from that of the whole soil, reflecting a smaller effect of OM in the sand fractions. These results suggested that OM contained in the clay fractions has a more important role on ion adsorption than that contained in the sand fractions and that fertilizer management will be more effective in soils with a higher clay content for both red Oxisols and Ultisols.