In the last years User Interface Description Languages (UIDL) appeared as a suitable solution for developing multi-target user interfaces. By applying appropriate model transformations, specifications of User Interfaces (UI) created with UIDLs can be reused and adapted according to constraints imposed by input/output devices, different contexts of use, or specific user preference. From a technical point of view transformational approaches can be used to adapt any aspect (i.e. structure, dialog and presentation) of user interfaces. In the practice, automatic adaptation of the dialog is not that easy. This paper discusses the role of the dialog modeling for the design of usable user interfaces and it proposes a design method which employs formal description techniques to deal with dialog modeling of user interface created with a UIDL called UsiXML.