There are two primary goals for this study – first, to analyse definiteness and article use in spontaneous writing in Swedish by 15-year-old Finnish immersion students (n=162) and secondly, to compare their performance with that of non-immersion students at the same age (n=67). Analyses at the group level show that immersion students usually perform significantly better than the control group, but they also reveal similar problems to what L2-Swedish non-immersion students have demonstrated in previous studies, such as omission of indefinite articles and difficulty in choosing the right definite form of the noun. Still, these inaccuracies occurred less often in the data from the immersion students. The studied constructions also show at the group level an acquisition order similar to that reported in previous studies, explainable by different aspects of complexity and cross-linguistic influence. Analyses on the individual level, however, show different acquisition orders depending on the criteria being used.