Usability is a main quality attribute for any interactive product. Usability in touch screen-based mobile devices is something essential and should be considered when launching a new product, it could be a distinguishing feature in a rushing market, as it is the one of the mobile devices nowadays. Traditional methods for usability measuring do not really fit the nature of these devices. There is a need for new usability evaluation methods or at least for the use of traditional evaluations in novel ways. A set of specific usability heuristics for touch screen-based mobile devices is proposed and (preliminary) validated.