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 degreesC. 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.