Reichardt's E-T(30) as well as Kamlet-Taft's alpha (hydrogen-bond-donor acidity) and pi* (dipolarity/polarizability) values of various solid acids, e.g., silicas, aluminas, alumosilicates, titanium dioxides, and alumina-functionalized silica particles, are presented. The E-T(30) values of the solid acids were directly measured by UV/vis spectroscopy using 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate (1a) and an eicosafluorine-. substituted derivative of 1a (1b) as surface polarity indicators. Kamlet-Taft's alpha and pi* parameters for the various solid acids are analyzed by means of Fe(phen)(2)(CN)(2) [cis-dicyano-bis(1,10-phenanthroline)iron(II)] (2) and Michler's ketone [4,4'-bis(N,N-dimethylamino)benzophenone (3) as solvatochromic surface polarity indicators. The chemical interpretation of the alpha and pi* parameters and the nature of the surface sites which they reflect are discussed. The correspondence of-the UV/vis spectroscopic results to those of the IR-sensitive probe benzophenone and the fluorescence probe pyrene (literature data) is also discussed. It is evident that the solid surface environments observed by the various indicators are moderately strong dipolar/polarizable (pi* = 0.38 to 1.04) and are fairly strong hydrogen-bond donors (alpha = 1.00 to 1.99). Theoretical E-T(30) values of solid acids are calculated by applying linear solvation energy (LSE) relationships using the independently measured alpha and pi* values of the solid acids according to E-T(30) = [E-T(30)](o) + a alpha + s pi*. The respective dependence of the alpha and pi* terms, expressed by the coefficients a and s, upon the measured ET(30) value for solid acids is discussed in comparison to related multiple LSE correlations known for well-behaved regular solvents and functionalized silicas. The results show in general that values of polarity parameters of strong HBD and dipolar surfaces are strongly influenced by the experimental conditions applied.