Olfactory rivalry can occur when a binary mixture is sniffed repeatedly, with one percept dominating then the other. Experiment 1 demonstrated olfactory rivalry using several new techniques. Experiments 2 and 3 examined whether participants can notice rivalry. Participants received trials composed of odor pairs: either a mixture followed by the same mixture; or a pure odor followed by the same pure odor. On some trials participants judged whether the two stimuli were the same or different, to see if they could detect rivalry. On other trials participants judged the quality of each odor, allowing us to determine whether rivalry occurred. We found evidence for rivalry when we compared reports of odor quality for one stimulus and then the other, but no evidence that participants could detect this change. These findings are consistent with the idea that people can experience olfactory illusions, but may not know they have occurred.