Web-based mindfulness-based cognitive therapy (eMBCT) has been found effective in decreasing fatigue severity in patients suffering from Chronic Cancer-Related Fatigue (CCRF). In web-based therapy, guidance from a therapist positively affects treatment outcome. So far, less is known about what kind of therapist behaviours contribute to treatment outcome. The present study aimed at 1) identifying therapist behaviours during eMBCT and 2) exploring whether these behaviours were correlated to a decrease in fatigue severity among patients. Qualitative content analyses were performed on 537 feedback e-mails from five therapists sent to 31 patients within a secured portal. Through content analyses, nine therapist behaviours were identified: emphatic utterances, probing self-reflection, informing, psychoeducation, task prompting, paraphrasing, task reinforcement, providing group context and alliance bolstering. Among these behaviours task prompting (19%), paraphrasing (16%) and task reinforcement (15%) were the most common. Linear regression analyses showed a significant association between informing and task prompting on the one hand and a decrease in fatigue severity on the other. Multivariate analysis indicated that informing and task prompting jointly explain the decrease in fatigue. These findings underline the importance for therapists to provide patients with sufficient information and to encourage them to do the exercises.