Hans-Joachim Dahms (Vienna) gives a talk at the MCMP workshop "Influences on the Aufbau" (1-3 July, 2013) titled "Rudolf Carnap and Wilhelm Ostwald". Abstract: When Rudolf Carnap started work on his dissertation Der Raum in Summer 1920 he also hosted a conference with some of his Jena friends in his Buchenbach home about „a system of the sciences“. Carnap proposed as starting point for that discussion a scheme and ideas developed by Wilhelm Ostwald (1853-1932), the nobel-price winning chemist (1909) and monistic philosopher. Ostwald is also men- tioned and discussed in the Aufbau. What Carnap might have attracted to Ostwald’s work, are the fol- lowing items of common interest: (i) the construction of international artificial languages for everyday use (like esperanto), but also for sci-entific purposes, (ii) monism and the unity of the sciences, (iii) theories of colour, and last but not least: (iv) ethics. After a brief discussion of these points I will conclude with a tentative answer to the question why Ost- walds influence on Carnap seems to have diminished in the decade since the Buchenbach conference.