We measured blur thresholds for square wave gratings modulated either along the red-green, the luminance or the yellow-blue colour direction. The modulations along the red-green and yellow-blue resulted in zero luminance variation. For all three observers the blur thresholds for red-green and luminance stimuli were very similar. The blur thresholds for yellow-blue were much higher. These results are consistent with the idea that the luminance and red-green channels do not differ in their inherent spatial resolution.