A study was carried out to investigate corrosion at the cement/casing interface for 13Cr casing steel. Cement simulated pore solution (CSPS) was developed by exposing cement pieces to 5% NaCl at 100 °C and 10 MPa in equilibrium with CO2. Pore solution was extracted from the cement pieces using a die press. Chemical analysis of the pore solution extract was performed and used for preparing CSPS for 13Cr corrosion testing. Corrosion tests were performed in CSPS at equilibrium with CO2 or CO2+H2S at 10 MPa at 4, 85, and 200 °C. Corrosion rates were determined using linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), and mass loss coupons. Addition of H2S at 4 °C increased the corrosion rate, while at 85 °C it slightly decreased the corrosion rate. Only a small effect from H2S addition was observed at 200 °C. Cyclic voltammetry (CV) results showed passivation/depassivation of 13Cr in CSPS at 4 and 85 °C, while a transition to active corrosion was observed at 200 °C. Surface analysis using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) found that both the coverage and complexity of the corrosion products formed increased dramatically with increasing temperature.