The hydrolysis of sucrose in an acid medium was used as a temperature indicator to measure holding temperatures in a continuous thermal-processing unit. From a theoretical analysis, target conversion (chi) and errors in both acid concentration and conversion measurements were found to affect significantly the accuracy of predicted temperatures, whereas errors in the determination of the fluid mean residence time did not show a significant effect. For different pasteurisation temperatures (70 degrees C < T < 86 degrees C) and flow rates (4800 < Re < 11,300), the acid concentration in the medium was adjusted so that different extents of reaction could be tested, as the reaction rate was found to increase exponentially with [H+]. Nitric acid solutions were circulated through the unit, a sucrose solution was continuously fed to the entrance of the holding tube and sucrose concentration was analysed at the half way part and at the exit of the holding tube. The temperature of the holding section was then estimated from the measured conversion. These results compared well with thermocouple measurements, with deviations of less than 4 degrees C for conversions between 0.4 and 0.7, whereas greater errors were obtained for both low and high conversions.