Abstract The acquisition of a sufficient vocabulary is crucial in order to learn a language. It is suggested that, to comprehend a text well, at least 95% of the words should be recognized and understood. Hence, it is of value to investigate different methods of learning new words. The study compares two different methods of learning new words in English, the translation method, i.e. learning unknown words by using L1 translations, and the explanation/synonym method, which means learning unknown words by using explanations and synonyms in L2. This was undertaken by inviting a group of pupils use the two methods and then comparing the results between the tests. The research question was to establish whether there are any differences between these two methods. The study aimed to investigate whether one of the methods shows better results than the other and, if so, to what degree. To achieve this, data was collected from students at the Swedish IM-program at the upper secondary school. In this case, all the participants had been diagnosed within the ASD-spectre. This investigation is of interest because the Swedish curriculum is reluctant to permit the use of translation between the learners’ L1 and L2. The analysis of the findings indicates a slight advantage for the explanation/synonym method. The difference is not so substantial that any certain and far reaching conclusion can be drawn as to which method is the most effective, especially with such a relatively small study. The indication is that both methods can be useful in teaching, particularly when combined with other vocabulary learning activities. Keywords: Vocabulary acquisition, English, ASD-spectre, IM-program