Two LED-UV based photochemical flow reactors have been compared. One was a commercially available LED-UV flow reactor designed for water disinfection or sterilization and the other one was a home-made LED-UV flow reactor designed for analogous purposes. The photochemical performances of two mentioned flow reactors working both at about 275 nm were evaluated using uridine actinometry through the determination of the pseudo-first order kinetics rate constant of uridine photolysis and through the measurement of the incident light I absorbed by the actinometer solution. From these data, the energy released to the solution by the LED-UV sources was determined. Furthermore, a third LED-UV reactor working at 360 nm was evaluated with uridine actinometer. As expected in the latter case uridine was not photolyzed and the reactor was found unsuitable for water disinfection.