Objective To investigate the effect of ferrous sulphate (FeSO4) as a chemical activator to hydrogen peroxide tooth bleaching gels on color change and treatment time. Methods One hundred and twenty extracted human upper premolars, hue A or B, were selected. One enamel-dentin specimen was prepared from each tooth. The specimens were divided into twelve groups (n=10), based on bleaching product, FeSO4 and treatment time. Three in-office bleaching products were used in this study: Opalescence® BoostTM, PolaOffice® and ZOOM!® WhitespeedTM. Each product was used per its specific protocol: 1: untreated control (C), 2: FeSO4 added, using the product’s specified treatment time (Fe100 %), 3: FeSO4 added, using approximately 75 % of the specified time (Fe75 %), 4: FeSO4 added, using approximately 50 % of the specified time (Fe50 %). The color was measured before and immediately after bleaching, using a VITA Easyshade® V spectrophotometer based on the CIE-L*a*b* color system. Data between experimental groups of each product were analyzed using one-way ANOVA and Tukey’s or Tamhane’s test (p=0.05). Results Color parameter (ΔL, Δa, Δb and ΔE) evaluation indicated no differences between the C, Fe100 % and Fe75 % of each product. Compared with the C group, only Δb of the ZOOM!® WhitespeedTM Fe50 % group was significantly different (p=0.001). No differences were found between the Opalescence® BoostTM and PolaOffice® C and Fe50 % groups with each material. Conclusion Adding FeSO4 was effective in reducing the treatment time of Opalescence® BoostTM, PolaOffice® and ZOOM!® WhitespeedTM by at least 25 %.