Transient seepage analysis in unsaturated soils requires the soil-water characteristic curves (SWCCs) and permeability functions. Very often, the soil-water characteristic of a soil at high suction range (>500 kPa) is obtained by extrapolating laboratory-determined SWCC at low suction range (0 to 500 kPa) using semi-empirical fitting methods. Relatively few measurements of SWCCs have been made on decomposed soils at high suctions. In this study, SWCC at low suctions of a completely decomposed tuff (CDT) in Hong Kong was measured using pressure plate apparatus, whereas the SWCC at high suctions was measured using the relative humidity (RH) control technique. Hence, the SWCC of CDT at "full suction range" is obtained by combining the measured data at low and high suctions. Comparisons and evaluations are then made with the extrapolated data using several existing semi-empirical fitting methods.