The effect of pH (over the range of 2.10 - 11.90) and temperature (from 353 to 363 K) on the stability of xylometazoline (XYL) has been studied by high performance liquid chromatography (HPLC). Using the dependence of log k vs pH, the hydrolysis of both the undissociated and dissociated XYL-molcules has been described by micro rate constants k(1) and k(2). Thermodynamic parameters (activation energy, enthalpy and entropy) of the composition reaction were calculated.