Introduction Internet-delivered cognitive and behavioral interventions have been shown to be effective in enhancing university students' well-being and reducing symptoms of stress and depression. However, few studies have examined the active components that may contribute to their effectiveness. The present study aimed to explore what kind of online written coach feedback would be useful on participants’ outcomes and satisfaction in two Acceptance and Commitment Therapy (ACT) blended interventions.

Methods ACT-trained master's level psychology students offered coach support (n = 50; 88% female, Mage = 26 years) to university students, who were randomly assigned to two parallel groups, each offered three face-to-face sessions and an online five-module ACT-based program. One group (iACTa, n = 61) received free-form personalized, individually tailored written feedback, and the other (iACTb, n = 62) received semi-structured written feedback with minimal personalization options.

Results Both groups had gains from participating in the interventions across all measures, including well-being, psychological flexibility, mindfulness skills, and reduction of stress and depression symptoms. However, there were no significant differences between the two groups except for well-being, which favored the iACTb. The iACTa group saw mostly small to large effect sizes (d = .44–0.80) while medium to large effect sizes were observed for the iACTb group (d = .69–0.83). The retention rate was good (iACTa: n = 51, 83.6%, iACTb: n = 54, 87.1%). Semi-structured features almost halved the coaches’ written response time for each participant (31 min in iACTa vs. 18 min in iACTb).

Conclusions The results shed light in the very limited research available and suggest that ACT-based, blended internet interventions with semi-structured coach feedback and minimally tailored features can be as effective as interventions with fully personalized feedback in treatment outcomes and participants’ satisfaction.