This study described the development and assessment of the psychometric properties of the Dissociation-Integration of Self-States Scale (D-ISS). This is a new scale to assess dissociation at the ‘between modes’ or self-state (personality) level. The D-ISS is rooted in cognitive behavioural theory and designed to measure between-mode dissociation (dissociation between self-states) in clinical practice and research.

Study 1: D-ISS scale items were generated and then answered by 344 young adults (16–25 years) who reported experiencing stressful times. An exploratory factor analysis (EFA) was conducted and the results were used to refine the scale to 25 items.

Study 2: The final 25-item D-ISS was completed by 383 adults (18–65 years) who reported experiencing mental health difficulties. A confirmatory factor analysis (CFA) was conducted using the second dataset. Internal consistency, test–retest reliability, convergent validity and divergent validity of the final D-ISS was assessed.

Study 1: The EFA showed a clear 5-factor solution, which was used to refine the D-ISS to a total of 25 items with five items in each factor.

Study 2: The 5-factor solution from Study 1 was confirmed as a good fit by the CFA using the data collected in Study 2. The D-ISS demonstrated good internal reliability and test–retest reliability. The D-ISS showed no correlations with divergent scales. For convergent validity, the D-ISS showed moderate correlations with the Dissociative Experiences Scale (DES-II).

The new D-ISS measure of between-mode dissociation is reliable and valid for the population represented by our sample. Further research into its use in clinical populations is required.

1. (1) To understand and be able to use a new measure of dissociation at the personality or self-states level.

2. (2) To understand the cognitive behavioural model of dissociation.

3. (3) To understand the theoretical underpinnings of the scale, in terms of the effects of childhood and adult adversity and other factors on psychological development.

4. (4) To consider the potential clinical and research applications of the scale.

5. (5) To appreciate the limitations of the research so far and the nature of future research required.