The interactive effects of salinity and water on organic matter decomposition in soil are poorly known. A loamy topsoil adjusted to five concentrations of salinity (0, 31, 62, 93 and 124 mmol Na kg(-1) soil) using either NaCl or Na2SO4 was incubated at a water content of either 17 or 25% (w/w) in the dark at 28.5 degrees C for 47 days, with maize straw added at 20 g kg(-1) soil. Comparing with non-saline soil, (1) NaCl salinity at all levels decreased cumulative CO2 evolved during days 1-3 (averaged across two water levels), increased in the period 4-32 days at both water contents, and thereafter caused variable effects, depending upon water content and salinity; and (2) Na2SO4 salinity at various levels mainly caused no effect on cumulative CO2 evolved during days 1-3 (averaged across two water levels), and thereafter (i.e. in days 4-47) caused mainly positive effects at 17% (w/w) water content and negative effects at 25% (w/w) water content. Cumulative CO2 evolved over 47 days for both types of salinities was mainly greater at 17% (w/w) and smaller at 25% (w/w) water content compared with non-saline soil. Generally, at 25% (w/w) than at 17% (w/w) water content, there was a greater CO2 evolved over 47 days, and also during different incubation phases for both types of salinities; the difference at low salinity levels was generally large and decreased as salinity increased. In conclusion, the salinity effect depends on soil water content and incubation period or decomposition phase.