AimsTo determine the degree to which the observed benefit of Text2Quit was accounted for by psychosocial mechanisms derived from its quit smoking messaging versus from the use of extra-programmatic smoking cessation treatments and services. DesignProspective, multiple mediation model of a randomized controlled trial (RCT). SettingUnited States nation-wide. ParticipantsA total of 409 adult daily smokers participated. Participants were, on average, 35years of age, predominantly female (68%), white (79%), lacked a college degree (70%), had medium nicotine dependence (average Fagerstrom Nicotine Dependence Score score of 5.2) and more than half (62%) had made a previous quit attempt. InterventionAdult daily smokers browsing the web for smoking cessation support (n=409; recruited 19 May2011-10 July 2012) were randomized to receive smoking cessation support via Text2Quit versus a smoking cessation material. MeasurementsMediators (i.e. changes in psychosocial constructs of health behavior change, use of extra-programmatic treatment) were assessed at 1 month using single-item measures and outcome (i.e. self-reported 7-day point prevalence abstinence) at 6-month follow-up. FindingsMediators accounted for 35% of the effect of Text2Quit on smoking cessation. Only psychosocial mechanisms had complete mediational paths, with increases in self-efficacy [b=0.10 (0.06-0.15)], quitting know-how [b=0.07 (0.03-0.11)] and the sense that someone cared [b=0.06 (0.01-0.11)], partially explaining the conferred benefit of Text2Quit. Use of outside resources, including treatments promoted explicitly by Text2Quit, i.e. medication [b=0.001 (-0.01 to 0.01), quitline [b=-0.002 (-0.01 to 0.04)], treatments and resources not promoted by Text2Quit, i.e. online forums [b=0.01 (-0.01 to 0.04)] and self-help materials [b=-0.01 (-0.04 to 0.02)], did not have complete mediational paths. An interaction effect existed for medication use that suggested that for participants not using medication, Text2Quit conferred substantial benefit, but not for participants using medication. ConclusionsText-messaging programs for smoking cessation appear primarily to confer benefit by promoting improvements in the psychosocial processes related to quitting rather than through the use of extra-programmatic smoking cessation treatments and services.