The Songkhram River, the third largest tributary of the Mekong, represents an important area of wetland biodiversity associated with the Mekong River system in tropical Southeast Asia. This area is endangered by human population growth and the associated spread of commerce and industry. Nevertheless, no adequate hydrochemical data are available to act as a baseline for further limnological studies. We carried out measurements of air and water temperatures, pH, conductivity, dissolved oxygen, salinity, total dissolved solids and transparency at six sites along the length of the river from July 2005 to May 2006. The ambient temperature ranged from 24 to 40 °C with an overall average of 29.4 °C, while the average water temperature is 28.2 °C and never exceeds 35.0 °C. The pH value is within the range of 5.3-7.4 (mean 6.11). There is a trend towards increasing pH in the dry season when the water level is low. Conductivity varies spatially and temporally among study sites. There is a trend towards increasing conductivity when the water level decreases, and conversely with increasing water level. Temporal and spatial variations in dissolved oxygen show a trend that suggests an increase as the water level decreases. The total dissolved solids show no variation either temporally or spatially in the wet season but vary considerably in the dry season. These data provide a sound basis for comprehensive future studies on the ecology of the Songkhram River. © The Japanese Society of Limnology 2008.