Background On 11 January 2009, Taiwan expanded its smoke-free legislation to all indoor public places and workplaces. This study examined the impact of this policy on secondhand smoke (SHS) exposure in adult non-smokers, across gender and socioeconomic status groups (SES). Methods An annual sample of about 13 000-14 000 non-smokers was drawn from cross-sectional nationwide data of Taiwan The model with interaction terms showed that men were more likely than women to be exposed to workplace SHS (OR=2.02, 95% CI 1.80 to 2.27) but were less likely to be exposed to home SHS (OR=0.79, 95% CI 0.73 to 0.86). SHS exposure in homes was significantly related to lower socioeconomic status, but the 2009 smoke-free policy reduced the difference in SHS exposure across education levels. Conclusions The 2009 smoke-free policy reduced the SHS exposure for non-smokers. However, this impact on home SHS did not persist after 2009, and the effect of protection was unequal across gender and SES groups. Thus, further enforcement of smoking restrictions would be needed to reduce the risk of SHS exposure and improve protection against SHS risk among parts of the population with lower socioeconomic status.