Orientation discrimination and tilt aftereffects (TAEs) were measured to determine if the orientation of luminance and illusory contours are processed by separate mechanisms. The assumption was made that if a single mechanism supports the perception of both types of contours, then illusory and luminance contours that support the same level of orientation discrimination will be equally effective adapting patterns. Experiment I found that orientation discrimination psychometric functions for illusory and luminance contours are similar, confirming that performance could be matched. Experiment II measured orientation discrimination for a range of intensities for both contours. Experiment III measured TAEs following adaptation to illusory and luminance contours that supported a similar range of orientation discrimination. Similar TAEs were not observed, thus rejecting the single mechanism hypothesis. Experiments IV and V sought to validate the assumption that equivalent orientation discrimination predicts equivalent TAEs by using stimuli that seemed likely to be represented by the same visual mechanism. Luminance contours masked by randomly placed dots and unmasked luminance contours were used with the same procedures as experiments II and III. Equal TAEs were not observed for masked and unmasked contours matched on orientation discrimination, suggesting the assumption relating discriminability to adaptation was incorrect.