According to the ease-of-processing hypothesis, judgments of learning (JOLs) rely on the ease with which items are committed to memory during encoding--that is, encoding fluency. Conclusive evidence for this hypothesis does not yet exist because encoding fluency and item difficulty have been confounded in all previous studies. To disentangle the effects of encoding fluency and item difficulty on JOLs, we used a variant of the learner-observer-judge method in which participants observed the study phase of another participant and indicated his or her JOLs. At the same time, the to-be-studied word pairs were concealed by strings of symbols. Our experiment revealed that participants use self-paced study time as a cue for JOLs when they themselves have studied and recalled word pairs before. This metacognitive monitoring of study time provides strong support for the ease-of-processing hypothesis. (Contains 2 footnotes and 1 figure.)