•Hydrolysis of sodium toluene sulfonate (STS) in sub-/supercritical water was investigated.•Supercritical water oxidation was used for total removal of STS from wastewater.•The optimum condition is T=452°C, water and CO2 flow rate of 2.3 and 0.08mLmin−1, respectively.•Hydrolysis of STS increases with the increase in temperature and CO2 flow rate. The hydrolysis of sodium toluene sulfonate (STS) in sub-/supercritical water in the presence of CO2 was investigated using a continuous flow system at the constant pressure of 250bar. The effect of water flow-rates, reaction temperature and CO2 flow-rates on the hydrolysis of STS was investigated. Results showed that hydrolysis of STS increases with the increase in temperature and CO2 flow rate. The optimum reaction conditions calculated by partial factorial design were 452°C, water flow rate of 2.3mLmin−1 and CO2 flow rate of 0.08mLmin−1. Furthermore, supercritical water was used for total removal of STS from industrial wastewater using H2O2 as an oxidant. Results showed that at temperatures higher than 350°C more than 99% of STS is oxidized while COD remains high until reaction temperature of 450°C after which COD of solution diminished 81%.