Relatives of people with psychosis or bipolar disorder experience high levels of distress but are typically not offered the support they need. Online peer forums may offer a solution, but knowledge about who uses them, how, and why is limited. This talk will describe online forum use during the Relatives Education and Coping Toolkit (REACT) trial. I will describe who used the forum and why; how sociodemographic factors are associated with participation; the relationship among frequency, type of use, and outcomes; and how the forum was used. A mixed methods approach was used to examine forum use combining quantitative explorations of key sociodemographic characteristics, levels of forum use, and distress and thematic and semantic analyses to understand the reasons for relatives joining the forum and the key topics they initiated. Although only a proportion of REACT participants engaged actively with its forum, they were widely distributed across age and other sociodemographic groupings. Relatives used the forum for information, support, and guidance and to offer detailed information about their experiences. The topics raised highlighted the burden carried by relatives and the potential value of easy-access, moderated, peer-supported forums in helping relatives to manage the challenges they faced. These findings are discussed in relation to their implications for future research and clinical practice.