The effect of the presence of acetic acid (HAc) on the formation of corrosion product layers in aqueous CO2 environments was investigated using electrochemistry, weight loss, and extensive surface analysis procedures. Electrochemical tests (LPR, EIS, and potentiodynamic sweeps) were conducted in a 4 L glass cell with seven specimens of API 5L X65 steel - one for electrochemical measurements and six for surface analyses. Experiments were conducted in test solutions without organic acid and with 0.5 mM (30 ppm) undissociated HAc for ~240 h (10 days) and specimens were retrieved after 12 h, 120 h, and 240 h. The condition necessary for precipitation of iron carbonate, i.e., saturation level S(FeCO3) >1 was achieved by adding an external source of Fe2+ ions (de-oxygenated aqueous FeCl2) at the beginning of the experiment. Modified thermodynamic calculations for the initial saturation value for iron carbonate, Sin(FeCO3), were used to compensate for complexation in experiments conducted in the presence of HAc. Surface and cross-sectional analysis were performed using XRD, SEM/EDS, and surface profilometry. It was observed that the corrosion product layers, formed without HAc and with 0.5 mM free HAc, remained protective, showing no indication of localized corrosion. The anodic and cathodic reactions of mild steel in solutions without HAc and with 0.5 mM free HAc were retarded by the formation of corrosion product layers indicating their protectiveness.