Firms engage in co-opetition (simultaneously competing and cooperating) behaviors when participating in Technology Standard Setting Organizations (SSOs). SSOs are organizations where competing firms and/or other technological institutions collaborate to develop standards for new technologies, such as Bluetooth, Wi-Fi, SSD, and 5G. Firms share their proprietary R&D information with competitors when cooperating to develop technology standards. I examine whether such co-opetition behaviors at SSOs alter firms’ incentives to publicly disclose narrative R&D information. I find that firms increase quantity and detailedness of narrative R&D disclosure after participating in SSOs. However, I find no evidence that firms adjust their disclosure of other non-R&D proprietary information (customer identity information) after participating in SSOs. The results imply that proprietary information can be multidimensional, and that empirical examinations of firms’ proprietary disclosure require the type of proprietary information to be relevant to the economic factor under examination. The main results of this study remain robust after using alternative SSO participation measures and using an instrumental variable approach. Finally, I provide evidence on the positive effect of SSO participation on increasing peer firms’ innovation efficiency.