Background <p>The Dissociative Experiences Scale (DES-II) is widely used globally. However, psychometric properties of the scale have not been adequately examined. The present study aimed to examine the psychometric properties and longitudinal stability of the DES-II.</p> Method <p>We collected data at two time points, approximately three and a half years apart. At Time 1 (T1), 1029 participants (515 females, 514 males) with a mean age of 44.64 (± 14.02) responded to the survey. Out of the T1 participants, 210 individuals (105 females, 105 males) also responded to the T2 survey. We conducted item parameters of the DES-II with item response theory (IRT).</p> Results <p>Our results showed that the DES-II is suitable for measuring strong dissociative traits, with all items displaying high discriminative power. The cut-off points for the DES-II were within a good range of measurement accuracy, and longitudinal stability over approximately three and a half years was adequate.</p> Conclusion <p>In the present study, we applied item response theory (IRT) to the DES-II, which has traditionally been interpreted using classical test theory (CTT). Results suggested the need for item-focused assessment rather than relying solely on mean scores or cut-off points. Specifically, results suggested that the severity levels differed across item ratings, and to set cut-off points for each item based on the severity of the ratings. Furthermore, the possibility of cultural differences in response patterns of the DES-II was indicated. However, few studies have discussed cultural differences based on IRT; hence, further research should examine response patterns of the DES-II across various cultures. In conclusion, the DES-II is a valuable tool for assessing dissociative symptoms, with adequate psychometric properties from an item response theory perspective. Clinicians should consider item-specific responses in their assessments, and further research is needed to explore the scale's applicability across diverse populations.</p>