Display omitted •Subcritical water is used as reaction medium to hydrolyze rutin into quercetin.•Optimization of reaction parametes is perfomed using response surface methodology.•The maximum obtained quercetin yield is 89.11±0.55%.•Addition of CO2 to subcritical water increases the amount of formed quercetin.•Higher amount of 5-HMF is formed from disaccharide with addition of CO2 to water. In this study, subcritical water has been used as a medium for hydrolyzing the glycoside bonded antioxidant quercetin-3-O-rutinoside (rutin) into its free aglycone form. Effect of temperature, treatment time, rutin concentration and the atmosphere used for establishing the pressure in the reactor was studied and the optimal combination of reaction parameters was established using response surface methodology in a 3×3 Box-Behnken design. Optimal reaction conditions were found to be a temperature of 196°C, time of 12.35min, rutin concentration of 1.08mg/mL and CO2 atmosphere at pressure of 215bar. Since high-temperature was applied, possibility of formation of hydrothermal degradation products existed, therefore total amounts of degraded quercetin and formed 5-hydroxymethylfurfural were determined. Results show that near the optimal conditions, practically all rutin is hydrolyzed. Quercetin degradation is unavoidable, although the effect can be minimized by increasing the CO2 pressure, but simultaneously that benefits to the formation of 5-hydroxymethylfurfural.