Biological and biomimetic apatites allow incorporations of many cationic and anionic substituents, namely Na+, CO32−, and potentially K+, that influence apatite's physicochemical, thermodynamic, and structural properties. Carbonate substitution can modify these properties depending on the CO32− type, such as hydroxyl (A-type) or phosphate (B-type) sites, or labile ions within the non-apatitic surface layer. While Na+ co-substitutions can affect CO32− integration, it is unknown if and how K+, a larger cation, may affect CO32− substitution and related apatite properties. Therefore, the purpose of this study was to compare the effects of K+ and Na+ on the physicochemical incorporation of CO32− in biomimetic apatites. To do so, 2–6 wt% CO32− apatites were precipitated in either Na- or K-rich solutions under optimized synthesis and maturation conditions. Atomic absorption data showed more Na+ substitution into the apatite structure than K+. FTIR and XRD results indicated that Na-apatites primarily contained B-type CO32−, while K-apatites included more A-type CO32− at higher total wt% CO32−. A deep FTIR analysis of the CO32− vibration modes showed that a red-shift occurred for the ν2CO32− and ν3CO32− B-type of the Na-apatites, suggesting a longer CO bond length, while K-apatites had a slight blue-shift for only ν2CO32−. Apatitic and non-apatitic HPO42− retention was also higher for Na-apatites than K-apatites. Together, these observations suggest that modification of the local CO32− environments depends on the monovalent cation. Overall, our data reveals the distinct mechanisms for Na+/CO32− and K+/CO32− co-substitution, which may shed light on the carbonation of biological apatites and their structural features.