Attachment theory posits that early relationships shape emotional development through Internal Working Models of self and others, reflected in attachment dimensions (anxiety and avoidance). These dimensions influence emotion regulation (ER) and the strategies used to manage emotions, with physiological manifestations, such as pupil dilation. This study investigates how attachment anxiety and avoidance interact with perceived ER success (PERCS) and emotional arousal to influence pupil size during cognitive reappraisal and expressive suppression. Eighty-five adults (ages 18-58) viewed emotional images from the International Affective Picture System, including both negative and neutral stimuli, while implementing these two ER strategies. Pupil diameter was recorded continuously from stimulus onset (0 ms) to 4,000 ms. Linear mixed-effects modelling revealed that reappraisal, compared to suppression, elicited greater pupil dilation, particularly between 500 and 4,000 ms post-stimulus. Attachment anxiety exhibited increased pupil dilation during reappraisal, reflecting heightened cognitive effort and hypervigilance, while attachment avoidance showed the opposite pattern, with reduced pupil dilation. Emotional arousal significantly predicted larger pupil size across conditions. Additionally, higher PERCS were associated with smaller pupil dilation during reappraisal. These findings emphasize the role of attachment dimensions in shaping physiological responses during emotional challenges.