— Our work focuses on the use of formal techniques in order to increase the quality of HCI software and of all the processes resulting from the development, verification, design and validation activities. This paper shows how the B formal technique can be used for user tasks modelling and validation. A trace based semantics is used to describe either the HCI or the user tasks. Each task is modelled by a sequence of fired events. Each event ned in the abstract specification and design of the HCI system.