SUMMARYThe charge characteristics of A1or Ap and B2horizon samples of total 23 Ultisols, Alfisols and Oxisols in Korea and Thailand were studied by measuring the retention of NH4+and NO3−at different pH values (4–8) and NH4NO3concentrations (0.1–0.005m). The magnitude of their negative charge (σ−; meq/100g) was dependent on pH and NH4NO3concentration (C;m) as represented by a regression equation: log σ−=apH +blogC +c. The values of the coefficienta(0.04–0.226),b(0.03–0.264) andc(–0.676–1.262) were correlated with the kinds of the soil and horizon and with the region where the soil exists. The retention of NO3−was less than 1 and 2–3 meq/100 g for the A1or Ap and B2horizon samples, respectively.The sum of exchangeable base and Al (‘effective’ CEC) was close to and higher than the magnitude of permanent charge (=σ−measured at pH = 4.3 and at C = 0.005m) for one‐third and two‐thirds of samples, respectively. A σ−value of 16 meq/100 g clay at pH = 7 and C = 0.01mwas found appropriate to separate the B2horizons of Thai Ultisols and Oxisols from those of Korean Ultisols and Alfisols. Korean Alfisols and Ultisols and Thai Ultisols were distinguished from each other on