The mutually influential interaction between Carnap and Neurath defined the then emerging left wing of the Vienna Circle in the early 1930s. Here it is investigated whether a comparable interaction can be detected already in the period leading to the publication of Carnap's Aufbau in 1928 when Neurath had been one of the readers of the typescript of an earlier version of the book that circulated amongst members of Schlick's discussion group. With reference to a brief note of Carnap's which recorded some his comments from this time, this paper investigates whether Neurath had any influence on the final version and what form this influence may have taken.