BACKGROUND: Multiple micronutrient (MMN) supplementation may result in interaction effects due to competing absorptive pathways of trace elements. OBJECTIVES: The aim of this study was to investigate the effect of MMN supplementation with or without iron on serum zinc, selenium, and copper concentrations in Cambodian women. METHODS: In a 2 × 2 factorial double-blind randomized 12-wk trial, predominantly anemic, nonpregnant women (aged 18-45 y) received daily 60 mg of iron (Fe; n = 201); 14 other micronutrients including zinc (15 mg), selenium (65 µg), and copper (2 mg), but no iron (MMN; n = 202); 60 mg iron plus MMN (Fe + MMN; n = 206); or a placebo (n = 200). Fasting morning blood was collected at baseline and 12 wk from women in 26 villages in Kampong Chhnang province. Serum zinc, selenium, and copper concentrations (secondary outcomes of the randomized controlled trial) were measured using inductively coupled plasma mass spectrometry. Generalized linear regression was used to estimate intervention effects [ß coefficient (95% CI)] for Fe (with or without MMN) and MMN (with or without Fe) after testing for the presence of an Fe × MMN interaction. RESULTS: A total of 760 women completed the trial. Zinc deficiency prevalence at baseline was 45% (inflammation-adjusted serum zinc <10.7 µmol/L). A significant Fe × MMN interaction (P = 0.02) was detected in the 2 × 2 analysis with serum zinc concentration as the outcome: the MMN group had a higher mean serum zinc concentration at 12 wk (12.3 µmol/L; 95% CI: 12.2, 12.4 µmol/L) compared with all other groups, and the Fe + MMN group had a higher mean serum zinc concentration (11.6 µmol/L; 95% CI: 11.5, 11.7 µmol/L) compared with the Fe group (11.0 µmol/L; 95% CI: 10.9, 11.0 µmol/L) and the placebo group (11.2 µmol/L; 95% CI: 11.1, 11.4 µmol/L). CONCLUSIONS: The inclusion of 60 mg iron in the daily MMN formulation may be interfering with the absorption and/or metabolism of supplemental zinc in Cambodian women. This is of particular concern when MMN supplementation is implemented in populations with risk of zinc deficiency. This trial was registered at clinicaltrials.gov as NCT-02481375.