The aim of this paper is to draw attention to a scientist who played an important role in Carnap's formative years (from the time he was 14 until at least the publication of the Aufbau): Wilhelm Ostwald, a Nobel-prize-winning chemist and leader of the Deutscher Monistenbund (as sucessor of its founder Ernst Haeckel). I will focus on three episodes: (a) Carnap's youth (before the First World War), (b) a meeting in Buchenbach in August 1920, which Carnap organized with some of his Jena friends, and (c) the Aufbau.I am very grateful to André Carus both for transforming my clumsy English text into something readable and for suggesting many valuable additions and clarifications.