The majority of experimental tobacco regulatory science (TRS) to date has focused specifically on the impacts of possible tobacco product standards (e.g. limit on e-cigarette flavoring) on the use of the single tobacco product targeted by the standard (e.g. e-cigarettes) among individuals who primarily or solely use the targeted product (e.g. vapers). Whereas this “single target” approach has yielded a large amount of actionable evidence, it has also resulted in a significant gap in the regulatory science evidence base for two interrelated reasons: First, the narrow focus on single tobacco product (STP) users has resulted in very little evidence on the impact of possible new product standards among multiple tobacco product (MTP) users who make up 30- 40% of the tobacco using population (10% of the U.S. adult population). Dual users of e-cigarettes (EC) and combusted cigarettes (CC) make up 40% of the MTP using population. Second, our preliminary evidence suggests that any two tobacco products vary in the degree to which they substitute for one another and that possible new product standards targeting one tobacco product may have an impact on the appeal and use of other tobacco products. In the proposed research, we will fill the gaps identified above by evaluating the effects of limiting EC e-liquid flavors to tobacco-only on the appeal of, and preference for, CC in a sample of adult dual users of EC and CC. In addition to evaluating the impact of limiting e-liquid flavor on the use of CC as they exist now (Aim 1), we will also examine the effects of limiting EC liquid on CC preference following standards that eliminate menthol in CC (Aim 2) and reduce CC nicotine content (Aim 3). We chose these possible policy actions as they have already been enacted at local levels, have been the subject of proposed rulemaking, or have been debated in the literature. In each experiment, adult dual EC/CC users will undergo preference sessions during which they will make choices between an EC and a CC. We hypothesize that restricting the flavor of EC to tobacco only, will increase preference for CC (Aim 1); that among menthol smokers, limiting EC to tobacco-only will increase choices for non-menthol CC relative to when EC are a preferred flavor, or when participants use their own EC device (Aim 2); and limiting EC to tobacco-only will increase choices for VLNC cigarettes relative to when EC are a preferred flavor, or the participant's own device (Aim 3). The proposed research will provide the FDA with actionable information on the impact of possible product standards for one tobacco product on the appeal and use other tobacco products. This research will be conducted among individuals who regularly use more than one tobacco product, and as such, are most vulnerable to changes in patterns of use of non-targeted products following changes in the characteristics of products targeted by new standards. Our aims are highly responsive to FDA research priorities as list...