Abstract: A system (93) monitors the break-up phase of an electrostatic deflection continuous ink jet printer to identify phase instability likely to be caused by a partial blockage of the jet-forming device (17). It ignores alternations between adjacent phase positions and brief unrepeated periods of phase disruption. Preferably it ignores phase changes caused by changes in other operational parameters of the printer, such as variations in ink pressure. Monitoring may be done in the printer (99) or in an external system (93), (95).