As a part of International Cooperative Studies on the Increasing Productivity of Soils in Tropical Area (1976-1978). chemical properties of paddy soils in Thailand and Malaysia were studied for the purpose of elucidation of the fundamental characteristics of the soils so as to evaluate soil fertility and to implement advanced technical practices for higher production of rice. Each several soil samples were collected from the central, northern, northeastern and southern regions of Thailand and the western Malaysia. Research in soil chemical analysis has led to findings which are of great significance to soil fertility. On the basis of analytical data fertility of the soils were evaluated, and soil series were classified into the three categories of low, moderate and high fertility according to the inherent fertility of their surface soils. Soils from the soil series of low and moderate fertility have many defective characters which should be ameliorated, such as low pH, low contents of available nitrogen, phosphorus and/or potassium and high salinity. However, it might be not so troublesome problems to ameliorate these soils, when the problems of water are resolved.