This paper analyses the contrasted role played by the concept of “inner form” in the structural and phenomenological perspectives on language of Gustav Špet, Roman Jakobson and Maurice Merleau-Ponty. On the basis first of a brief overview of the concept’s antecedents in the theories of Wilhelm von Humboldt and the Humboldtian psychological tradition (Steinthal, Potebnja, Wundt, Marty), and then a comparative analysis of the general function of inner form in the works of Špet, Jakobson, and Merleau-Ponty, the paper suggests that inner form was a catalyser in the formulation of a new, structural and phenomenological conception of expression. The productive ties of expression with inner form are demonstrated positively in the cases of Špet and Jakobson, and by negative contrast in the case of Merleau-Ponty. Where Špet and Jakobson formulate a triadic model (outer form – inner form – content) whose middle term is then identified as primary and as the bearer (or function) of language’s expressivity (i.e. its capacity to express meaning through its particular verbal form), Merleau-Ponty remains bound to a binary opposition between form (or style) and content that condemns him to think of expression not as a functional property of language but in terms of a constant movement of subjective distanciation (écart) and reappropriation.