The use of the Self-reporting Tool On Pain in people with Intellectual Disabilities (STOP-ID!), an online application developed by the authors to aid in the self-reporting of pain, was evaluated in 40 adults with Down syndrome. Comprehension of the use of the tool (the ability to recognize representations for vocabulary and pain, and to navigate the tool interface), and the use of the tool to self-report pain experience, were investigated. The use of the online tool was investigated with both a laptop and a tablet computer in a crossover design. The results provide evidence that more participants recognized representations of pain location and pain affect than representations of pain intensity and pain quality. A small percentage of participants demonstrated the ability to recognize all of the representations of vocabulary items and to navigate the tool without assistance (18% laptop, 18% tablet). Half of the participants were able to report at least one pain component of a current or remembered pain experience without assistance (50% laptop, 53% tablet). Ways to improve the design of tools for reporting pain and to improve performance are suggested.