Background: Internet-delivered cognitive behaviour therapy (iCBT) has been found to be an effective way to disseminate psychological treatment, and support given by a therapist seems to be important in order to achieve good outcomes. Little is known about what the therapists actually do when they provide support in iCBT and whether their behaviour influences treatment outcome. Aims: This study addressed the content of therapist e-mails in guided iCBT for generalized anxiety disorder. Method: We examined 490 e-mails from three therapists providing support to 44 patients who participated in a controlled trial on iCBT for generalized anxiety disorder. Results: Through content analysis of the written correspondence, eight distinguishable therapist behaviours were derived: deadline flexibility, task reinforcement, alliance bolstering, task prompting, psychoeducation, self-disclosure, self-efficacy shaping, and empathetic utterances. We found that task reinforcement, task prompting, self-efficacy shaping and empathetic utterances correlated with module completion. Deadline flexibility was negatively associated with outcome and task reinforcement positively correlated with changes on the Penn State Worry Questionnaire. Conclusions: Different types of therapist behaviours can be identified in iCBT, and though many of these behaviours are correlated to each other, different behaviours have an impact on change in symptoms and module completion. © British Association for Behavioural and Cognitive Psychotherapies 2012.