Abstract: An apparatus and method for use in an optical recording system in which an incident beam records data on an optical recording medium as marks and in which a return beam reflected from the medium is detected in an optical detector to provide a reflected write signal. A number of reflected write pulses of the reflected write signal are monitored in an interference detector. The peak voltage or other suitable characteristic of a reflected pulse corresponding to a mark following a relatively short land is compared to the same characteristic of another reflected pulse corresponding to a mark following a longer land. An error signal is generated based on the comparison and provides an indication of the amount of intersymbol interference (ISI) present during recording of the marks corresponding to the first and second reflected pulses.