This study examines how immigrant parents' geographic origins correspond to their adult children's ethnic and racial self‐classification; whether discrepancies are associated with socioeconomic status; and the implications of these patterns for assessing socioeconomic inequality. Using linked British census data, we identify immigrants' children in 1971 and examine how they ethnically/racially self‐classify in 2001. We find that fluidity in classification varies across groups, but higher educational attainment is consistently associated with less white British classification. Therefore, grouping immigrants' children by ethnic/racial self‐classification underestimates socioeconomic disadvantage for these groups. However, grouping by parental birthplace overlooks variation in racialization and disadvantage among children of immigrants from the same country of origin.