The purpose of this paper was to do an investigation of students’ writing in order to find out if students in upper secondary school were capable of writing for different purposes, i.e. if they used more formal features in formal writing than in informal writing. I also wanted to find out if the students made fewer errors in a formal text than in a less formal text. The results are based on an analysis of two types of texts produced by thirteen upper secondary school students. One of the text types was a short story about what it is like to move away from home and the other text type was a formal letter to a travel agency. The texts were carefully read in order to detect any grammatical differences and the focus was on some selected grammatical features which are typical in formal and informal texts. The results showed no distinct grammatical differences between the two text types. The students used formal features like nouns, if-clauses and prepositional phrases equally much in both texts. However, Nouns of French or Latin origins, which are commonly used in formal registers, had a higher frequency in the students’ formal letters than in their informal texts. In addition, contractions, which are very informal and commonly used in conversations, appeared little in both texts. Further, the results showed that some students did manage to create a more official tone in the formal letter, which indicated that some students may change registers depending on what audience they have. The results also showed that the students made the same type of errors in both texts. Sentences begun with And and the wrong tense were used in the informal texts as well as in the formal texts. Nyckelord: Register, style, informal writing, formal writing, linguistics.