Quite often, adult language learners are already familiar with one or more second languages at the point at which they start learning a new one. This type of situation is probably becoming even more frequent in the present-day society, due to several factors: an increase in travelling and working abroad, greater focus on languages in education, greater exposure to other languages through the media, and so on. Prior L2 knowledge is often mentioned as a source of influence on the acquisition of a new language. Despite this, there have as yet been very few comprehensive studies carried out in order to investigate the influence of previously learned second languages on the new language being learned (cf. the bibliography by Henriksson and Ringbom 1985, and the survey of previous research in Ringbom 1987: 112ff). This situation may be due to an assumption that the influence of prior L2 knowledge is generally weak or insignificant, or the difficulty in finding suitable situations in practice where the interaction between prior L1 and L2 knowledge and the developing new language can be studied.

In the following, we will distinguish between the first language (L1), prior second languages (L2s), and the language that the learner is currently acquiring, which we will label the third language (L3). We will present an outline of a longitudinal case study of one learner's acquisition of Swedish as a third language, and give some examples to illustrate a few, as yet very preliminary findings.