Rudolf Carnap was an Esperantist who also had an active interest in other international auxiliary languages (IALs), and noted a psychological affinity between the construction of IALs and symbolic language systems. This paper provides a detailed reconstruction of Carnap’s engagement with Esperanto and other IALs, drawing on his underdiscussed essay on IALs, “The Problem of a World Language,” and his unpublished material from the Virtual Archive of Logical Empiricism (VALEP). It addresses two questions: the first is about the relationship between (...) his interests in the two forms of language engineering, IALs and symbolic logic, and the second is about the philosophical implications of his engagement with IALs. As for the first question, I argue that underlying Carnap’s interests in both projects is the common idea that natural language is unsuited for some special purposes, which are better served by constructing new special languages. As for the second, I argue that considering Carnap’s views on IALs reveals that despite a utopian tendency, his view of language has several realistic aspects, such as (1) non-perfectionism, (2) experimentalism, and (3) a recognition of the role of psychological factors in language choice. Additionally, this paper also highlights Carnap’s neglected concerns about linguistic injustice suffered by speakers of minor languages and the use of the generic masculine. (shrink)