Previous studies have shown that 180 degrees relative phase shifts in f + 2f gratings are discriminated when the cosine or the sine component of the shift exceeds some criterion (Bennett & Banks, 1987; Field & Nachmias, 1984). The current experiments demonstrate that this result holds for other two-component gratings, provided that the components are within two to three octaves of each other. For frequency differences greater than two to three octaves, phase-reversal discrimination is impossible. A simple model that discriminates phase shifts on the basis of changes in the responses of even- and odd-symmetric spatial filters can account for the results.