If the target in a crowding display differs from the distracters in its contrast polarity, the extent of crowding is reduced compared to the condition where all the elements in the display have the same polarity. In Experiment 1, we test the temporal properties of this polarity advantage by reversing the contrast of the target and flankers at four frequencies between 2 and 15 Hz. In the same polarity condition, target and distracters were all white in one frame but all black in the next. In the opposite polarity condition, the target was white and distracters black in one frame and all reversed in the next frame. Less crowding was seen for the opposite polarity condition at lower frequencies, but this advantage disappeared at 7.5 Hz and higher frequencies. In Experiment 2, we test whether this result can be explained by lateral masking, using a display that matched the crowding configuration. Lateral masking did not exhibit a polarity advantage at any frequency. Hence, the polarity advantage in crowding, and its loss at 6-8 Hz, cannot be attributed to lateral masking. It is known that attention has a coarse temporal resolution (6-8 Hz). The findings of this study suggest a role for attention in crowding, as opposed to low-level mechanisms like lateral masking.