Abstract: Degree of opening of heterocyclic ring of imidazolinium inhibitors at temperatures higher than 25 ℃ due to hydrolysis was determined by UV-spectrophotometry at the wavelengths of 205 nm and 228 nm,which were the absorption maxima of amides,the hydrolysate of imidazoline and of the imidazoline ring itself respectively.Measurements of absorbance values of the test solutions were made at the respective wavelengths after hydrolysis for 4 h at elevated temperatures and cooling the solution to room temperature (25 ℃).Concentrations of the hydrolysate (c1) and of the imidazoline remained (c2) were found from the calibration curves with the measured absorbance values,and degree of ring-opening was calculated by the formula given in the paper.Ranges of linearity of the calibration curves for determination of both the hydrolysate and the imidazoline were same and in the range of 5-50 mg·L-1.