During the course of routine high-performance liquid chromatographic analyses of brain catecholamines using dual-electrode electrochemical detection, we encountered an unusual negative peak in the lower-voltage channel. Subsequent investigations suggested that this peak was caused by tyrosine which produced a positive peak in the higher-voltage channel. Our investigations indicate that compounds that generate a peak in one channel appear to be responsible for complex peaks in a second channel set at a lower voltage, close to or below that necessary for oxidation. The complex peaks are biphasic; a sharp negative peak coinciding with the positive peak on the higher-voltage channel, followed by a positive peak. This effect was not specific for tyrosine, but was observed on the lower-voltage channel with all compounds tested that produced signals on the high-voltage channel. The cause of the problem is unknown, but it appears to be an artifact of the electrical coupling of the two electrode channels in a dual-channel system.