Cognitive control has been typically examined using single-item tasks. This has implications for the generalizability of theories of control implementation. Previous studies have revealed that different control demands are posed by tasks depending on whether they present stimuli individually (i.e., single-item) or simultaneously in array format (i.e., multi-item). In the present study, we tracked within-task performance in single-item and multi-item Stroop tasks using simultaneous pupillometry, gaze, and behavioral response measures, aiming to explore the implications of format differences for cognitive control. The results indicated within-task performance decline in the multi-item version of the Stroop task, accompanied by pupil constriction and dwell time increase, in both the incongruent and the neutral condition. In contrast, no performance decline or dwell time increase was observed in the course of the single-item version of the task. We interpret these findings in terms of capacity constraints on cognitive control, with implications for cognitive control research, and highlight the need for better understanding of the cognitive demands of multi-item tasks. (PsycInfo Database Record (c) 2023 APA, all rights reserved).