Catalytic constants have been measured for the hydrolysis of a range of aryl orthocarbonates, in which both the leaving group and trioxocarbenium ion moiety have been systematically varied, by neutral carboxylic acids (trichloroacetic, difluoroacetic, dichloroacetic, malonic, chloroacetic, and acetic) at 70.0 degrees C in 60% water-40% acetonitrile, I = 1.0 M (KCI). Curvature cannot be detected in Bronsted plots involving carboxylic acids only, but inclusion of the point for H3O+ suggests downward curvature (i.e. p(x) greater than or equal to 0). beta(lg), Plots are curved downward (i.e. p(y’) = -partial derivative beta(lg)/partial derivative pK(lg) > 0). Substitutent effects in the tris(aryloxy)carbenium ion fragment were quantitated by use of the experimental aqueous pK(a) values of the phenol (pK(rc)), rather than Hammett sigma values, since this gave better correlations for the spontaneous reactions (Kandanarachchi, P.; Sinnott, M. L. J. Am. Chem. Sec., preceding paper in this issue). Cross coefficients are large and not constant : p(xy’) (measured as partial derivative alpha/partial derivative pK(lg) rather than partial derivative beta(lg)/partial derivative pK(HA)) varies from 0.26 for (PhOC+ to 0.16 for (pMeOC(6)H(4)OC+. Likewise, p(xy) (measured as -partial derivative alpha/partial derivative pK(rc) rather than -partial derivative beta(rc)/partial derivative partial derivative pK(HA)) experiences large changes with the leaving group pK. Data to estimate p(yy’) (beta(rc)/partial derivative K-lg) are more limited, but it too changes with the pK(a) of the catalyzing acid. These data indicate that a two-dimensional More O’Ferrall-Jencks diagram, with one axis representing both C-O cleavage and the ability of substituents in the tris(aryloxy)carbenium ion moiety to sense positive charge development, is inadequate to represent this reaction : separate axes are required for carbon-oxygen bond cleavage and development of carbon-oxygen double-bond character.