Expanding on our earlier building block architecture [(MeO)₂CH–Linker–Pro–X–NHNH₂ where X = Phe, Cha], we have produced a series of new pseudo-dipeptides [(MeO)₂CH–Linker–Pro–X–NHNH₂ where X = Val, Leu, Ile, Ala] for use in hydrazone-based dynamic combinatorial libraries (DCLs); reverse order analogues [Phe-Pro and Val-Pro] and two enantio-analogues [Pro-Phe and Pro-Val] were also prepared. The behaviours of these building blocks in DCLs, as single components and in mixtures, were studied systematically using HPLC and mass spectrometry in order to gain insight into the relationship between building block structure and good library diversity. Subtle changes in building block structure lead to significant changes in library distribution and in the ability to produce diverse libraries in mixtures.