The moisture characteristics and related pore size distributions of Thai Oxisols were determined for 11 representative pedons. These soils are Kandiustox and Kandiudox located in the North-east Plateau, South-east Coast, and Peninsular Thailand. They are generally acidic (pH 5-6), clayey, have low cation exchange capacity (5-15 cmol(c)/kg) and have low bulk densities (0.77-1.36 Mg/m(3)). The microstructures of these soils are mostly granular with compound packing 10-1500 mu m voids between aggregates. Water retention curves for all soils are similar with amounts of water retained at 33 kPa (field capacity) ranging from 22 to 43 (% weight) and 1500 kPa (permanent wilting point) ranging from 17 to 34%. Water available to plants is 3-12%. There is a strongly bimodal pore size distribution with the modal macropore and micropore diameters calculated from the soil moisture retention characteristic curves being about 100 mu m and 0.02 mu m. The pore size distribution determined by N-2-BET method indicates a modal micropore of about 0.01 mu m. The total volume of pores >5 mu m estimated by image analysis of scanning electron micrographs ranges from 20 to 50% with most of the variation in porosity being due to large differences in macropore (>75 mu m) volume.