This article studies the places and modes of intervention of values in argumentative discourse. In classical rhetoric, the question of values is central in the epideictic genre, whereas logos is privileged in the forensic or deliberative context. Nevertheless, the texts often appear to be a hybrid between the various genres. Such phenomena of “permeability” between the epideictic and deliberative genres are illuminated by the analysis of letters sent for publication in the “letters to the editor” section of a regional daily paper in a debate focusing on the construction of a third airport in the Parisian region. The appeal to values which underlies the texts places them primarily in the epideictic register. The function usually attributed to the epideictic species is that of consolidating social cohesion. Beyond that, such an appeal to values is also reinvested in the opposition to the project of the third airport, in view of the imminence of a decision-making that subordinates the speeches to a concrete deliberative purpose. Finally, these epideictic displays can arouse reactions mobilizing strategies deployed to escape the essentially oratorical tone of the appeal to values, and to integrate it into an argumentative critical speech where logos becomes central again.