The aim of this work was to design a simple millireactor system and to apply a sensor film to the surface of the channel. The sensor film was prepared by the sol-gel method. In this work it was studied whether a functional pH sensor film can be obtained which will give a clear response in real time inside the channels of the millireactor. To obtain the sensor film, tetraethoxysilane, TEOS, and phenyltrimethoxysilane, FTMS, were used as precursors for the sol-gel method, ethanol as the solvent, methyl red as the pH indicator and HCl as the reaction catalyst. The resulting sensor films were first tested on plates and then applied in milireactor channels. Examination of the plates on the tiles concluded that the excessive thickness of the layer leads to cracking of the films, which causes the leaking of the indicator and the film itself to separate from the surface. Therefore, thinner films were obtained in several methods. The first method was to reduce the preparation time of the mixture to ensure a lower viscosity during application, and the second was to dilute the mixture by increasing the amount of solvent, ethanol. By analyzing the functionality of the plates, it was found that the first method, without the addition of more ethanol, gives the best sensor films and therefore this film was selected for application in milireactor channels. The tests in the milireactor channels were performed with a peristaltic pump, by the cyclic exchange of acid and alkali using rinsing with distilled water in between the cycles. Within a time interval of 5 minutes the results show a change in the color of the sensor depending on the pH value of the solution. Change of color is very difficult to see with the naked eye and requires RGB color analysis software. It can be concluded that it is necessary to further optimize the obtained sensor films in order to give a clearer response in real time.