In the present study, advanced learners’ knowledge of L1 and L2 vocabulary is investigated and compared. Eight parallel L1 and L2 tests were administered focusing on vocabulary from upper secondary school level, specialised uses of vocabulary, advanced vocabulary, word formation with an emphasis on suffixation, idioms/proverbs, idiomatically used prepositions/multi-word verbs, polysemous words/lexical fields of near synonyms/false friends and inferencing. 15 first-term university students taking English as a single subject course were included as informants. One native speaker, whose results are primarily used as a point of reference, was also incorporated.

     In the first chapter of the book differences in L1 and L2 learners’ vocabulary breadth and depth are discussed. Setting the stage for the rest of the investigation, based on the learners’ results on vocabulary from upper secondary school level, specialised uses of vocabulary and advanced vocabulary, the chapter also offers insights into the participants’ L1 and L2 (as compared to the native speaker) vocabulary size.

     In Chapter 2 word formation, especially suffixation, is in focus. Among other things, the results here show that the success or non-success with which the learners were able to provide derivative forms in their L2 depends on a complex interplay between the frequencies of the stem and derivative, the complexity of the suffix and the relative frequencies of the stem and suffix. Correlations could here also be seen between the learners’ vocabulary size and their derivation skills in that a sizeable knowledge of words generally also contributed to high scores on the suffixation tests.

     Chapter 3 deals with the learners’ mastery of idioms and proverbs. While the frequencies of the test items here had no effect on whether the meaning of an idiom was understood or not, the degree of transparency played a great role in the students’ L2, the most transparent items being understood the best. The results also show that the learners, in order to figure out the meanings of the idiomatic expressions, very often tended to resort to the context in which they were presented. All in all, idiom comprehension in an L2 seems to be a much more heuristic approach than idiom comprehension in an L1.

     The fourth chapter investigates the students’ mastery of on the one hand prepositions that are used idiomatically (e.g. for in reason for and on in comment on) and on the other hand multi-word verbs (e.g. cut in (=interrupt)). Vast differences in knowledge were here detected in the students’ L2 as compared to in their L1, the scores being very low in the learners’ second language. Also, while the knowledge of the verb and particle forming a multi-word verb appears to go hand in hand, this was not the case with the idiomatically used prepositions. Put differently, whereas the verb and the particle in multi-word verbs seem to form tightly knit units in learners’ mental lexicons, the same cannot be said for combinations such as reason for and comment on.

     Chapter 5 focuses on the learners’ mastery of polysemous words, lexical fields of near synonyms and false friends, i.e. it deals primarily with vocabulary depth. While it was the number of meanings sought that was the determining factor as to whether an L2 polysemous word was known or not, frequency was seen to be an important predictor of success with items in lexical fields of near synonyms, not only within lexical sets but also to some extent among different sets of items. Also, false friends in general caused great problems, the learners here achieving a very low average score.

     The last chapter is concerned with the learners’ inferencing skills. It turns out that the participants made extensive use of contextual clues and did so quite successfully, generally more so in their L1 than in their L2. Correlations could here also be seen between the students’ knowledge of breadth and depth and their inferencing skills so that those learners who produced the best results with the former type of knowledge were also those that did best on the inferencing tests.