Java is becoming more and more important in various communities. It is widely used for developing classical, distributed and real time applications. One of its key features in these domains is its multithreading system. Although a specification exists, it is informally written in the English language. Therefore, an additional formalization effort is required. This paper focuses on that point. Our aim is to provide a model that can be reused and help in the processes of: using Java threads to gain deep knowledge of their behavior; designing new threading systems taking the best out of Java threads still avoiding their main drawbacks; proving properties ‐ this is for instance what we need in other research projects carried out in our team on automatic distribution of objects. The model that we have set up uses transition systems. To check that it corresponds to the informal specification, we use the MEC model checker. Also, we use MEC to automatically check the properties we are interested in. For example, we use it to exhibit a known problem of the Java threading system: the handling oflong anddouble variables.