English words consist of three parts: a root, a prefix, and a suffix. A prefix usually changes the meaning of a word whereas a suffix normally only changes the category of the word while retaining its meaning. Knowing about derivational morphology may improve students’ ability to read, and make it much more efficient. The purpose of this paper was to investigate how advanced students of English as a second language use derivational morphemes and what type of derivation, prefixing or suffixing, was more prevalent in students’ free writing. The material for the investigation consisted of 40 essays from a total of 20 university students. I used two essays written by the same student, one from the A- and one from the B-level. The results of this investigation showed that the students in the investigation used altogether 6 different prefixes and 36 different suffixes, where the most prevalent derivation by far was suffixing. There was an increase in the use of derivational morphemes from the A-level to the B-level. The investigation also showed that more than half of the students included in my study improved in their way of using different affixes as well as their frequency of use of them, which indicates that derivational morphology develops along with language proficiency. Nyckelord: Derivation, morphology, prefix, root, suffix, reading comprehension, language proficiency.