Understood as a question of co-operative justice, the issue of linguistic justice concerns the way in which the learning costs of a lingua franca must be shared between the various linguistic groups that benefit from the communication potential created by the existence of this lingua franca. The article approaches this question by first considering three prima facie plausible criteria to be found in the literature — economic, sociolinguistic and philosophical, respectively. Next it formulates putatively decisive objections to each of these within the framework of a simple example with two linguistic groups and proposes two further criteria, indistinguishable in this simple framework, which it then generalises to the general case of n linguistic groups. In this more general framework, one of these two criteria turns out to be blatantly untenable, while the other seems to survive the challenge of potentially counter-intuitive implications, at any rate as long as one bears in mind the distinction between co-operative and distributive justice.