Feedback literature identifies mindful cognitive processing of (peer) feedback and (peer) feedback characteristics--as well as the presence of justifications for feedback--as important for its efficiency. However, mindful cognitive processing has yet to be operationalized and investigated. In this study, an operationalization of mindful cognitive processing is introduced, alongside an investigation to identify valid measures for it. In a between-subjects design, peer feedback (PF) content [elaborated specific feedback with justifications (ESF?+?J) vs. elaborated specific feedback without justifications (ESF)] was varied. Students received a scenario containing an essay by a fictional student and fictional PF, followed by a text revision, distraction and PF recall task. Eye tracking was applied to measure (a) how written PF was (re-) read (fixation durations) and (b) the number of transitions occurring between PF and essay text. Mindful cognitive processing was inferred from the relation between fixation durations on PF and number of transitions between essay text and PF with (a) text revision performance and (b) PF recall performance. When no justifications were provided, recipients invested more time in reading the PF and essay and increased the effort to relate the PF to essay text. Fixation durations and number of transitions proved to be valid measures to infer mindful cognitive processing.