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Phosphoric acid etching before internal bleaching may enhance peroxide penetration. The objective of this ex vivo study was to compare the whitening efficacy of four treatment protocols: 35% hydrogen peroxide with (HP + E) and without (HP) etching, and 35% carbamide peroxide with (CP + E) or without (CP) etching. Twenty-four extracted teeth were allocated to one of these treatment protocols. Treatments were applied for 21 days with renewal at 7 and 14 days. Samples were stored in a humidity chamber at 37°C. Tooth shade was measured weekly using an electronic spectrophotometer, after agent rinsing. Overall shade change (∆E00) and Whiteness Index for Dentistry (WID) were calculated. Two-way ANOVA and Tukey's HSD test were used to analyze differences between groups. After 21 days, HP + E was found as the most effective bleaching agent (∆E00 = 4.98 ± 1.09), statistically superior to CP and showing a strong tendency towards superiority over HP and CP + E. ∆WID scores were consistently higher for HP + E than for CP groups. Only HP + E reached clinical significance in both ∆E00 (>2) and ∆WID (>3) for all samples after 21 days, thus showing strong reliability. These data support recommendation for clinicians to use phosphoric acid etching prior to internal bleaching, especially for treating most discolored teeth.