At the start and the heart of every successful Solid-Phase Peptide Synthesis (SPPS), a top-grade starting resin (i.e. the solid support) is needed. However, in contrast to well established test methods for amino acid derivatives, solvents and reagents, meaningful and discriminating test methods for solid supports are hard to develop because of the insolubility of such resins. This presentation will be focused on the characterization of the solid supports e.g. polystyrene and pre-loaded amino acid resins. Physico-chemical quality attributes e.g. mesh size distribution, extent of polystyrene cross-linking, swelling properties and their impact on SPPS results will be discussed. In addition, spectroscopic (i.e. IR, NMR and UV/Vis) data to characterize starting resins will be presented. Those test methods were developed and are routinely used by Bachem.