The DEMMI (de Morton Mobility Index) is a new and advanced instrument for measuring the mobility of all older adults across clinical settings. It overcomes practical and clinimetric limitations of existing mobility instruments. This study reports the process of item reduction using the Rasch model in the development of the DEMMI. Prior to this study, qualitative methods were employed to generate a pool of 51 items for potential inclusion in the DEMMI. The aim of this study was to reduce the item set to a unidimensional subset of items that ranged across the mobility spectrum from bed bound to high levels of independent mobility. Fifty-one physical performance mobility items were tested in a sample of older acute medical patients. A total of 215 mobility assessments were performed. Seventeen mobility items that spanned the mobility spectrum were selected for inclusion in the new instrument. The 17 item scale fitted the Rasch model. Items operated consistently across the mobility spectrum regardless of patient age, gender, cognition, primary language or time of administration during hospitalisation. Using the Rasch model, an interval level scoring system was developed with a score range of 0 to 100.