Reichardt's E-T(30) and Kamlet-Taft's alpha (hydrogen bond donor acidity), beta (hydrogen bond acceptor basicity), and pi* (dipolarity/polarizability) values for various poly(ol-amino acids) [-NH-CH*(R)-CO-](n) (PAs) and alpha -amino acid crystals [H3N+-CH*(R)-COO-](n) (AACs) are presented. The ET(30) values of the solid poly(a-amino acids) are directly measured by UV/vis spectroscopy using Reichardt's dyes la and Ib as surface polarity indicators. Kamlet-Taft's alpha, beta, and pi* parameters for the various solid PAs and AACs are analyzed by means of Fe(phen)(2)(CN)(2) [cis-dicydnobis(1,10-phenanthroline)iron (2), Michler's Ketone [4,4'-bis(N,N-dimethylamino)benzophenone] (3), 4-aminobenzophenone (4), and an aminobenzofurandione dye (5) as solvatochromic surface polarity indicators. The surface polarity of AACs is determined by a quite strong hydrogen-bond-donating capacity(alpha = 0.6-0.96) and moderate dipolarity/polarizability (pi* = 0.3-0.7). For the same substituent R, in general for PAs the a value is lower than the pi* value, whereas for AACs the result is opposite. Increasing the size of thd alkyl substituent R of PAs or AACs decreases the a and beta values. It can be shown that the beta value of solid PAs is a function of Taft's sterical parameter (E-s)-if the size of R decreases, the beta value increases-indicating a strong influence of the steric effects of R on the accessibility of the peptide bond to hydrogen-bond-accepting probes. Theoretical ET(30) values for AACs are calculated by applying linear solvation energy relationships using the independently measured alpha and pi* values of the solid acids according to E-T(30) = [ET(30)](0) + alpha alpha + s pi*, because E-T(30) values cannot be directly determined for AACs.