The article focuses on semi-auxiliaries which are noun-based semi-fixed phrases, and which, from a semantic point of view, express a modal value. Examples: avoir la possibilité de (to have the possibility to), se trouver dans l’obligation de (to be under the obligation to), se faire un devoir de (to make a duty of ), avoir l’honnêteté de (to have the honesty to), avoir le plaisir de (to have the pleasure to)... After specifying the status and role of these multiword expressions within verbal periphrases, and recalling the criteria for classifying modalities, we propose a classification of these phrases according to the modalities they express. This allows us to reveal the principles that guide their interpretation.