To assess the effects on brand appeal of plain packaging and size of pictorial health warnings (PHWs). Three (30%, 70% and 100% size front-of-pack PHWs) by two (branded versus plain) between-subjects online experiment. Australia. A total of 1203 adult smokers. Rating of cigarette brands, smoking attitudes and intentions, purchase intent. Compared to branded packs, plain packs reduced smokers' ratings of 'positive pack characteristics' (P < 0.001), 'positive smoker characteristics' (P < 0.001) and 'positive taste characteristics' (P = 0.039). Plain packs were rated as being smoked by people who were more 'boring' than those who smoked branded packs (P = 0.001). By contrast, increasing size of PHW above 30% only reduced ratings of 'positive pack characteristics' (P = 0.001), but also decreased ratings of smokers as being 'boring' (P = 0.027). Plainness and size of PHW interacted in predicting ratings of 'positive pack characteristics' (P = 0.008), so that when packs were plain, increasing the size of PHW above 30% did not further reduce ratings. Presentation of only plain packs increased the likelihood that smokers would not choose to purchase any pack (20.3%) compared to presentation of only branded packs (15.3%) (odds ratio = 1.4; P = 0.026), while size of PHWs had no influence upon purchase choice. Plain packaging probably plays a superior role in undermining brand appeal and purchase intent to increasing health warning size. Policymakers should not rely solely upon large health warnings, which are designed primarily to inform consumers about smoking harms, to also reduce brand appeal: both strategies are likely to be required. © 2012 The Authors, Addiction © 2012 Society for the Study of Addiction.