Socioeconomic status (SES) is often used as control variable when relations between academic outcomes and students' migrational background are investigated. When measuring SES, indicators used must have the same meaning across groups. This study aims to examine the measurement invariance of SES, using data from TIMSS, 2003. The study shows that a latent SES variable has the same meaning across sub-populations with Swedish and non-Swedish background. However, the assumption of scalar invariance was rejected, which is essential for estimation of differences in latent means between groups. Comparisons between models assuming different degrees of scalar invariance indicated that models allowing partial scalar invariance should not be used when comparing latent variable means across groups of students with different migrational backgrounds. (Contains 6 tables and 2 figures.)