This report presents the formal semantics of COALA, a design language based on the concept of Coordinated Atomic Actions (CA actions). COALA has been developped with the intent of providing a concrete language to express CA actions and thus facilitate the desing of softwares whose structure relies on this concept. The semantics is expressed using the formal object-oriented specification language CO-OPN/2. The complete semantics is given in the appendix; specifications were checked (syntax and types) using the tools developped for CO-OPN/2.