Rate constants for quenching of the triplet states of 15 organic compounds by copper(II) bis(2,4-pentanedionate), Cu(acac)(2), copper(II) bis( 1, 1, 1,5,5,5-hexafluora-2,4-pentanedionate), Cu(hfac)(2), Ni(II) bis(2,4-pentanedionate), Ni(acac)(2), and Ni(II) bis(1,1,1,5,5,5-hexafluoro-2,4-pentanedionate), Ni(hfac)(2), in acetonitrile solution have been measured using the nanosecond laser flash photolysis technique. The quenching data were interpreted in terms of energy or electron transfer alone or in terms of both as competitive processes. With a classical framework for energy and electron transfer, the correlations of the quenching rate constants with the standard free energy changes for energy and electron transfer were used to evaluate appropriate intrinsic barriers and transmission coefficients for both processes. In acetonitrile, quenching by Cu(acac)(2) was suggested to occur mainly by electron transfer, whereas quenching by Cu(hfac)(2) was shown to involve energy transfer to the ligand-localized triplet in combination with electron transfer. Applying both the "quadratic" Marcus and the "asymptotic" Agmon-Levine free-energy relationships led to similar values of intrinsic barriers and to low values of transmission coefficients for the electron-transfer processes studied. The nonadiabatic character of those processes was also confirmed using the recently developed semiclassical approach by Tachiya and Murata which allowed for variable electron-transfer distance. A quantum mechanical model was also applied to the data within the phenomenological kinetics scheme, and the fitting parameters were consistent with the other approaches. Quenching by the NI(II) 1,3-diketonates was adequately described by energy transfer to metal-centered excited states or ligand-localized triplet states. The influence of changing of solvents from benzene to acetonitrile on the quenching was also discussed.