The necessity of express control of the state of liquid media in real time is substantiated. Various methods of express control of the state of liquid media are considered. The basic requirements for these methods are determined. An optical method for monitoring the state of liquid media using the phenomenon of refraction is presented. The use of a small-sized differential-type refractometer for express control of the state of liquid media is substantiated. Its design has been developed and a new principle for measuring the refractive index of the investigated liquid has been proposed. The results of experimental studies are presented.