This study examines Flemish teenagers’ perceptions and evaluations of linguistic accommodation in instant messaging: people adapting their language use to (that of) their interlocutor. We conducted a survey among 254 pupils in Belgian secondary education, and compare the results to teenagers’ online writing practices in a reference corpus. Our findings yield insight in the indexicality of accommodation, with (more) mirroring evoking (more) positive evaluations of the relationship between interlocutors. As for self-reported mirroring, the socio-demographic variable of education stands out: while the participants report a frequent online contact with people who differ from them regarding age, gender, or educational track, they only report to accommodate in the first two situations. Furthermore, they perform quite poorly at recognizing accommodative adjustments made by their peers, especially for mixed-education interactions. Finally, while teenagers appear to hold largely the same opinions on accommodation, the analyses reveal some socio-demographic variation.