The design of an output multimodal system is a complex task due to the richness of today interaction contexts. The diversity of environments, systems and user profiles requires a new generation of software tools to specify complete and valid output interactions. In this paper, we present a multimodal output specification and simulation platform. After introducing the design process which inspired this platform, we describe the two main platform’s tools which respectively allow the outputs specification and the outputs simulation of a multimodal system. Finally, an application of the platform is illustrated through the outputs design on a mobile phone application. Categories and Subject Descriptors H.5.m [Information Interfaces and Presentation (e.g., HCI)]: Miscellaneous. General Terms Human Factors, Design. Keywords Human-Computer Interaction, output multimodality, outputs specification, outputs simulation.