Objective: In vitro comparison of the efficacy of two bleaching procedures, one based on carbamide peroxide (CP) and the other on hydrogen peroxide (HP), simulating clinical conditions. Study Design: Two groups of 20 teeth in each group were selected. Group A: 22% CP, one hour a day for 21 consecutive days. Group B: 37.5% HP, in 2 treatment sessions with an one week interval between each session. At each session the product was applied three successive times for eight minutes. Colour was recorded before treatment, when it was finished and one week after finishing it, with the Vita EasyShade spectrophotometer. CIEL*a*b* and ?E were established at each moment in the study. Intra-group data was compared using the paired t-test and inter-group data with the independent groups t-test. Scores from the Vita Classical guide provided with the device were recorded and the colour improvement percentage was calculated. Results: In both groups significant whitening was achieved by the end of treatment. Lightness remained significantly high when treatment was finished and one week after in both groups. The percentage of bleaching was significantly higher in group A. Conclusions: Both 22% CP and 37.5% HP were effective for bleaching teeth. Bleaching effect was greater in CP group. © Medicina Oral S. L.