Two independent liquid chromatographic critical ranges were identified in systems composed of the non-end-capped silica gel C-18 column packing, poly(methyl methacrylate)s (PMMAs), and mixed eluents containing acetonitrile (ACN) and toluene at 35 °C. ACN suppressed surface adsorption of PMMA on free silanols and promoted enthalpic partition of polymer species in favor of a solvated C-18 bonded phase. On the contrary, toluene promoted adsorption of PMMA on silanols and suppressed its enthalpic partition in favor of the C-18 phase. Changing the eluent composition, it was possible to separately control both the above enthalpic retention mechanisms. Consequently, a critical adsorption range was reached at one eluent composition with a negligible or only small effect of enthalpic partition, and vice versa, compensation of enthalpic partition and exclusion retention mechanisms was attained in eluent-suppressing adsorption. In this way, the existence was indirectly proved of surface adsorption and enthalpic partition as two similar but independent enthalpic retention mechanisms operative in high-performance liquid chromatography of polymers. In many cases adsorption and enthalpic partition act concurrently in a synergetic or an antagonistic way, and this may strongly affect the selectivity of polymer separation.