The purpose of this study was to investigate the mechanism of action of a bleaching agent, as it relates to enamel and dentin. Twenty-six extracted human molar teeth were sectioned at the cemento-enamel junction and were randomly assigned to two groups. L*a*b* readings were taken with a spectrophotometer: on buccal surfaces of the crown, at enamel and dentin. The teeth were exposed to carbamide peroxide or placebo gel and L*a*b* scores were again recorded to determine color changes. Treatments were compared using ancova test with baseline color as the covariate. Relative to placebo, buccal surfaces exhibited the greatest Delta b* and Delta L* color change. On buccal surfaces, the adjusted mean (SE) treatment differences were -7.8 (1.00) for Delta b* and 5.7 (0.97) for Delta L, with groups differing significantly (p < 0.0001). On enamel surfaces, treatment differences were -3.6 (0.61) for Delta b* and 4.6 (0.80) for Delta L* (p < 0.0001). Dentin exhibited the least color improvement. Adjusted mean (SE) treatment differences were -1.9 (0.87) for Delta b* and 2.4 (1.10) for Delta L*, with groups differing significantly (p < 0.02) on dentin color change. The majority of color change seen on the buccal surface of tooth crowns exposed to carbamide peroxide 15% was because of the color change in enamel. As compared to enamel, dentin was less affected after 14 days.