This study looks at the usability of mobile Twitter applications running on Android and Blackberry operating systems. Usability is important for mobile Twitter applications, as 55% of active Twitter users access the social network service through their mobile devices. This study gathers empirical data that determines which mobile Twitter application is superior in terms of usability. The mobile Twitter applications evaluated and compared in this study are the official Twitter applications for Android and Blackberry and the third-party Twitter application: TweetCaster for Android and Blackberry. Data is gathered using the System Usability Scale and notes regarding time spent on tasks and the accuracy of task completion are kept, in order to measure satisfaction, efficiency, and effectiveness respectively. The findings of this study show that there is need for further studies and highlights further areas for improvement on mobile Twitter applications.