Substantial harm could result from concurrent cigarette and e-cigarette use (i.e., dual use) were it to undermine smoking cessation. Perceptions of chemical exposure and resulting harms may influence dual use. We conducted a probability-based phone survey of 1164 U.S. adult cigarette smokers in 2014-2015 and analyzed results in 2016. In a between-subjects experiment, smokers heard a hypothetical scenario in which cigarettes and e-cigarettes had the same amount of harmful chemicals or cigarettes had more chemicals than e-cigarettes (10x more, 100x more, or chemicals were present only in cigarettes). Smokers indicated how the scenario would change their interest in dual use and perceived health harms. Few smokers (7%) who heard that the products have the same amount of chemicals were interested in initiating or increasing dual use. However, more smokers were interested when told that cigarettes have 10x more chemicals than e-cigarettes (31%), 100x more chemicals than e-cigarettes (32%), or chemicals were present only in cigarettes (43%) (all p < .001). Individuals told that cigarettes have more chemicals were more likely than those in the "same amount" scenario to perceive that cigarettes would be more harmful than e-cigarettes (79% vs. 41%, OR = 5.41, 95% CI = 4.08-7.17). These harm perceptions partially explained the relationship between chemical scenario and dual use interest. Smokers associated higher chemical amounts in cigarettes versus e-cigarettes with greater health harms from cigarettes and thus expressed increased interest in dual use. The findings suggest that disclosing amounts of chemicals in cigarette smoke and e-cigarette aerosol could unintentionally encourage dual use. (C) 2016 Elsevier Inc. All rights reserved.