This paper presents a luminescence sensor for real-time pH measurements. The emission spectra of mixture consisted of tested and base solution of fluorescein is used for analysis of the pH of aqueous solutions. The proposed microfluidic mixing system with integrated optical fibers allows pH measurement in the range 7.6 – 2.8. The developed sensor can be used in automatic pH control systems that work with aqueous solutions. The optimization of fluorescein concentration in base solution allowed to obtain linear characteristic of sensor response. The practical realization of the sensor with continuous monitoring of the pH of liquids and its potential applications are presented.