This article presents the features, design and architecture of the Myst pervasive game platform that has been applied in creating pervasive mobile learning games in various contexts such as science festivals and museums in Finland. Based on our experiences with the development, we draw a set of design principles for creating successfully a pervasive game platform that can be easily ported to various contexts. These principles advocate openness, flexibility, interaction models, connections to the outside world, and participatory design of the game content. In the evaluation part we present preliminary results of tests conducted in Finland at the SciFest 2008 festival in Joensuu and at the Museum of Technology in Helsinki. The results suggest that games built with the Myst platform are particularly suitable for children and young adults, and these games motivate players to interact with the environment and help to learn by discovering new things. The Myst platform has clearly potential for similar success in other environments due to easy portability and extensibility. (Contains 4 tables, 5 figures and 1 note.)