For the last 80 years, the Stroop task has been used to test theories of attention and cognitive control and it has been applied in many clinical settings. Most theories posit that the overwhelming power of written words overcomes strict instructions to focus on print color and ignore the word. Recent evidence suggests that trials in the Stroop task could in fact be a mixture of reading trials and non-reading trials. Here we conduct a critical test of this mixture hypothesis, where a mixture of processes should satisfy the fixed-point property (Falmagne, 1968)