Understanding the mechanisms of scale inhibitor (SI) retention in carbonate formations is key to designing efficient SI "squeeze" treatments in oil reservoirs. By performing "apparent adsorption" experiments, this paper demonstrates that a coupled adsorption/precipitation (Gamma/Pi) retention mechanism dominates in calcite and limestone for two widely applied scale inhibitors, DETPMP and PPCA. Precipitation was a more dominant retention mechanism at both initial pH values (pH(0) 4 and 6) and T = 80 and 95 degrees C for both SIs. At 95 degrees C, the pure adsorption (Gamma) region only extends up to [SI] similar to 100 ppm, above which precipitation (Pi) dominates. At lower temperatures (T = 80 degrees C), the solubility of the SI-M2+ complex increases, resulting in less precipitation. The apparent adsorption results are supplemented by measuring the corresponding solution [Ca2+], pH values in solution, and environmental scanning electron microscopy/energy dispersive X-ray analysis (ESEM/EDX) and particle size analysis (PSA), which give us a full mechanistic explanation of our results. For DETPMP, the retention increased as the solution pH increased, while retention of PPCA increased as the test pH decreased. Moreover, DETPMP was retained more than PPCA due to their differences in chemistry. Furthermore, the retention of both SIs was greater for the limestone sample due to Fe2+ traces enhancing the precipitate of SI-M2+.