Internet delivery of couple interventions is becoming increasingly popular; however, little is known about mechanisms of change during these interventions. One online, self-help relationship intervention—the OurRelationship program—has been shown to improve relationship satisfaction (Doss et al., 2016) during the intervention and to maintain gains through 12-months follow-up (Doss, Roddy, Nowlan, Rothman, & Christensen, 2019). This study seeks to understand mechanisms during and following this program using the same sample of 300 couples (600 individuals) randomly assigned to the program or a waitlist control group. Results from the bivariate growth curves revealed that greater increases in relationship satisfaction during the intervention were explained by improvements in negative communication, emotional intimacy, as well as target problem confidence and severity. However, changes in acceptance of targeted relationship problems, positive communication, and self-protective orientation were not related to changes in satisfaction during the intervention. Additionally, in a multivariate model, relationship target problem severity alone remained significant in the presence of other mechanisms. Improvements in positive and negative communication during the intervention and level of negative communication at the end of the intervention predicted maintenance of gains in satisfaction over 12-month follow-up. Results replicate previous findings that communication (Doss, Thum, Sevier, Atkins, & Christensen, 2005) and emotional intimacy (Doss et al., 2005; Hawrilenko, Gray, & Córdova, 2016) are key components in initial decreases in relationship distress. Furthermore, results suggest that improvements in communication may help couples more effectively navigate problems following the intervention—above and beyond its associations with prepost improvements in satisfaction.