The constitutive influence of Poincaré, Duhem and Rey on the philosophy of the Vienna Circle, long obscured, has become more widely recognized. Two aspects of the Viennese reception of these “French Conventionalists” are explored here for the light they may throw on the Circle’s own, still insufficiently understood conventionalism. First, what was the Viennese perception of what has recently been called (Poincaré’s) “structural realism” ? What if any part of that doctrine became assimilated into their theories ? Second, in the absence of a realist interpretation of the conventionalists structuralism, how where the principles guiding theory construction and validation to be legitimated ? It will be suggested that the left Vienna Circle developed a decidedly constructivist version of conventionalism in response.