Apparatus (2) for monitoring chlorine in water (4), which apparatus (2) comprises a flow cell (6), the flow cell (6) containing a chlorine sensor (62) and an automatic calibration system, and the automatic calibration system being such that it comprises a small pair of in-line electrodes (50) for generating low concentrations of free chlorine, and injection means (44) for injecting in use of the apparatus a salt solution (64) of known concentration.