The molecular shape recognition differences between monomeric and polymeric C18 stationary phases in the reversed-phase liquid chromatography (RPLC) separation of unsubstituted polycyclic aromatic hydrocarbons (PAHs) and methyl-substituted polycyclic aromatic hydrocarbons (MPAHs) are examined through the use of partial least squares (PLS) analysis techniques. The resulting PLS models are able to describe the enhanced shape selectivity of the polymeric phase for recognizing subtle structural differences among planar and nonplanar isomers. PLS component analyses of these models reveal that spatial and topological descriptors are primarily used to rank structural differences among the PAHs (i.e., fused-ring patterns, molecular length and breadth) that control such shape-selective chromatographic processes. This is consistent with the view that polymeric alkyl chain stationary phases contain size- and shape-specific "slots" that promote the separation of structurally-related solutes. In contrast, the monomeric phase model is limited in resolving both the isomer classes and the nonplanarity shape differences among the PAHs. However, an improvement of shape recognition on the monomeric phase was elucidated by the PLS model for two PAHs (phenanthro[3,4-c]phenanthrene and dibenzo[g,p]chrysene) exhibiting the most extreme nonplanarity. These results suggest that a limited amount of space between alkyl chains may exist within the higher-density polymeric phase to recognize shape differences among the bulkier and nonplanar solutes.