Many adolescents and adults with Down syndrome have reduced speech intelligibility. Reasons for this reduction may relate to differences in anatomy and physiology, both of which are important for creating an intelligible speech signal. The purpose of this study was to document acoustic vowel space and articulatory working space for two adult speakers with Down syndrome who had reduced speech intelligibility (mean = 56% based on single words). Articulatory data for the tongue were collected using a real-time flesh-point tracking method (i.e. X-ray microbeam). Results show smaller F1-F2 acoustic vowel space area for both speakers with Down syndrome compared with the control speakers. Reduced articulatory working space area and slower movement speed were also found for three of the four tongue points analysed. Although generalizations are limited by the small number of participants, findings warrant further investigation of the underlying articulatory characteristics of speech production for individuals with Down syndrome. (Contains 4 tables and 3 figures.)