Background: Internet-delivered cognitive behavioral therapy (ICBT) is an efficacious treatment for anxiety and depression. Most ICBT programs include therapist assistance in the form of secure online messaging; however, a high degree of variability has been found in the content of client and therapist correspondence. Recent research conducted by Svartvatten et al. (2015) found that client statements suggesting alliance bolstering and text expressing a positive change in mood after the implementation of a suggested skill or exercise appear to correlate with greater symptom improvement. Purpose: The current study sought to examine: (1) if previously identified themes in client communication with their Internet therapist (Svartvatten et al., 2015) would replicate in a transdiagnostic ICBT program for depression and anxiety; and (2) if these themes correlated with symptom improvement and treatment completion. Method: The present study used data from 80 randomly selected patients from a previously published trial of ICBT for depression and or anxiety. Client emails (on average 5.69 per client) were examined for the presence of 10 themes reported by Svartvatten et al. (2015). Results: Statistically significant differences were found in the frequency of all themes between the two studies. Further, in the current study, greater frequency of statements classified as maladaptive repetitive thinking and problems with treatment content correlated with smaller improvements in symptoms of anxiety from pre- to post-treatment. Limitations: Different material was presented to clients in the current study compared to clients in Svartvatten et al.’s (2015) study. Implications: This research provides a better understanding of the parameters of client communication and information for future therapists regarding the content of clients’ correspondence in ICBT.