Capacitive deionization (CDI) is an electrochemical desalination technology that removes ions from brackish water through electrosorption within charged porous electrodes. Brackish waters contain ions that may form precipitates on the electrodes (e.g., Ca2+, Mg2+, CO32−), especially in response to localized pH increases common at cathodes. Here we study the formation of calcium carbonate (CaCO3(s)) on the cathode and its effect on Na+ removal using defined solution chemistries. We found that CaCO3(s) formed during the charging phase at the cathode, but not the anode. Local precipitation was likely due to the in situ basic pH caused by the reduction of dissolved oxygen at the cathode. The deposition of solids reduced Na+ adsorption capacity by 67 % (0.94 ± 0.08 mg/g-C) compared to a control where no Ca2+ and HCO3− were present. We attributed this reduction primarily to the ~30 % decrease in micropores available for electrosorption. These interior pores were blocked by non-porous CaCO3 crystals. Cathodes with precipitation had reduced capacitance and increased resistance. The deposition of non-conductive crystals hindered the transfer of charge and restricted the movement of ions from the bulk solution toward the electrode surface and also through the pores' structure. Since precipitation was found to be detrimental to salt removal, future research is needed to reduce scaling and/or strategies to regenerate the electrosorption capacity of electrodes. For other applications, such as water softening, research to improve precipitation might be desired.