A group of nine phenylynamines (PhC drop CNH2, PhC drop CNHCH(CH3)(2), PhC drop CNHC6H11, PhC drop CNHC6H5, PhC drop CNHC6F5, PhC drop CN(CH2)(5), PhC drop CN(CH2CH2)O, PhC drop CN(CH2CH2CN)(2), and PhC drop CN(CH3)C6F5) were generated in aqueous solution by flash photolyic decarbonylation of the corresponding phenylaminocyclopropenones, and the kinetics of their facile decay in that medium were studied. This decay is catalyzed by acids for all ynamines--primary, secondary, and tertiary-and also by bases for primary and secondary ynamines. Solvent isotope effects and the form of acid-base catalysis show that the acid-catalyzed path involves formation of keteniminium ions by rate-determining proton transfer to the beta-carbon atoms of the ynamines. The ions generated from primary and secondary ynamines then lose nitrogen-bound protons to give ketenimines, and the ketenimines obtained from secondary ynamines are hydrated to phenylacetamides, whereas that from the primary ynamine tautomerizes to phenylacetonitrile. Keteniminium ions formed from tertiary ynamines have no nitrogen-bound protons that can be lost, and they are therefore captured by water instead, and the amide enols thus produced then ketonize to phenylacetamides. The base-catalyzed decay of primary and secondary ynamines also generates ketenimines, but protonation on the beta-carbon is now preceeded by proton removal from nitrogen. Rate constants for beta-carbon protonation of PhC drop CNHCH(CH3)(2) and PhC drop CN(CH2)(5) by a series of carboxylic acids give linear Bronsted relations with exponents alpha = 0.29 and 0.28, respectively, whereas inclusion of literature data for protonation of PhC drop CN(CH3)(2) by a group of weaker acids gives a curved Bronsted relation whose exponent varies from 0.25 to 0.97. Application of Marcus rate theory to this curved Bronsted relation produces the intrinsic barrier Delta G(0) double dagger = 3.26 +/- 0.19 kcal mol(-1) and the work term w(s) = 8.11 +/- 0.15 kcal mol(-1).