BackgroundIn internet-delivered cognitive behavioural therapy (ICBT) programs, beyond standardized core ICBT lessons, brief additional resources are sometimes available to clients to address comorbid concerns or offer additional information/strategies. These resources remain understudied in terms of how they are selected and perceived by clients, as well as their relationship to satisfaction and outcomes. MethodsAmong clients (N = 793) enrolled in a 5-lesson transdiagnostic ICBT course, we examined client use and perceptions of 18 additional resources at 8 weeks in terms of whether clients found resources informative (yes/no) and or helpful (yes/no). Resources elaborated on cognitive strategies (managing beliefs, risk calculation) or on managing specific problems (agricultural stress, alcohol misuse, anger, assertiveness, chronic conditions, communication, grief, health anxiety, motivation, pain, panic, postpartum depression/anxiety, PTSD, sleep, workplace accomodations, worry). Clients also completed symptom measures and ICBT satisfaction questions at 8 weeks. ResultsApproximately 50 % (n = 398) of clients rated the resources and, on average, clients reported that 3.35 (SD = 3.34) resources were informative and 2.35 (SD = 2.52) resources were helpful as measured by direct questions developed for this study. Higher pre-treatment PTSD and GAD scores were related to a greater number of resources perceived as informative and or helpful. Rating more resources as informative and or helpful had a weak but positive association with ICBT satisfaction and depression, anxiety, PTSD and insomnia change scores. Limitations of the study include that 31 % (n = 245) did not respond to questions about use of resources and 18.9 % (n = 150) said they did not review resources. ConclusionsThere is considerable use of diverse additional resources in ICBT in routine care. Associations suggest that clients are using resources to personalize treatment to their needs and these resources are associated with treatment satisfaction and outcomes. The correlational associations between symptoms and perceived helpfulness of resources can help inform personalization algorithms to optimize ICBT delivery for clients. Further research on how to match clients with, encourage use of, and maximize benefits of resources would be beneficial.