Objective Socioeconomic circumstances are critically important to addressing smoking. In young adulthood (ages 18–25), dynamic transitions in education, employment, family and housing complicate the measurement of socioeconomic circumstances. To better understand approaches to capturing these circumstances, this methodological systematic review examined how socioeconomic characteristics used to identify social inequalities in smoking among young adults are measured.

Data sources We searched PubMed/MEDLINE, Scopus, EMBASE, ERIC and Sociological Abstracts, and used three prior reviews. We updated the search in March 2018.

Study selection Two reviewers independently screened peer-reviewed records from OECD countries published in English, French, German or Spanish after 1995 whose samples covered at least 1 year between the ages of 18 and 25. We included 89 of 1320 records.

Data extraction One reviewer extracted study characteristics, indicators used to operationalise socioeconomic circumstances and each indicator’s relation to results on smoking (ie, significance and direction). We found 39 indicators of socioeconomic circumstances related to six broad domains. These indicators were used in 425 results.

Data synthesis We descriptively analysed the extracted data using evidence tables. Educational attainment was most common. Evidence of inequalities varied by indicator used. For example, there was inconsistent evidence regarding the role of parental characteristics and transition stages and insufficient evidence regarding personal income on smoking.

Conclusion Despite its importance, studies have disproportionally examined inequalities among young adults using traditional indicators. The mismatch between young adults’ life transitions and measurement strategies may attenuate evidence of inequalities. We suggest strategies to improve future measurement.