Objective: The aim of this study was to evaluate the effect of bleaching on the elution of monomers from two modern composite materials. Methods: Two different resin composites (a nanohybrid the Filtek Supreme™ XT and an ormocer the Ceram X™) were bleached with two products (hydrogen peroxide 38% for 45 min and carbamide peroxide 15% for 56 h). Four groups (n = 10, diameter: 4.5 mm, thickness: 2 mm) of each material were fabricated, two for each bleaching product. One group was used as unbleached control and the other one was bleached. Then the samples were stored in 1 ml of 75 vol% ethanol at room temperature, and the storage medium was renewed after 24 h, 7 days, and 28 days. From the storage medium that was removed samples were prepared and analysed with LC-MS/MS. Results: None of the bleaching products had an effect on the amount of Bis-GMA and TEGDMA released from Ceram X™. The amount of Bisphenol A released from the bleached samples of Ceram X™ was significantly lower compared to the control samples. Bleaching reduced significantly the amount of Bis-GMA released from Filtek Supreme™ XT. The amount of TEGDMA released from Filtek Supreme™ XT was not affected by bleaching. Significance: The bleaching agents tested in the present study reduced the amount of some of the monomers released from the two composite materials. Bleaching of modern composite materials does not increase the release of monomers. © 2008 Academy of Dental Materials.