This paper presents an evaluation of three designs for on-screen keyboard layouts for use by Single Switch Users (SSUs). SSUs are those users who have a disability which means that they are able to activate a switch into an "on" or "off'' position but are not able to locate or position a particular switch. The first keyboard layout design was alphabetically based, the second design was based upon the most frequently occurring letters in the English alphabet and the third design was the standard QWERTY layout found on most keyboards. The research shows that there is no significant difference in words per minute (WPM) for first time users between the three layouts. The researchers believe there was a significant learning effect for users going from one layout to the next and are of the opinion that further research on the layouts with experienced rather than first time users will yield interesting results. The overall aim of the research is to investigate different screen layouts with the goal of finding a screen layout best suited to SSUs.