A detailed kinetic study of the protonation and subsequent benzene elimination reactions of a (diimine)Pt-II diphenyl complex (denoted as (N-N)PtPh2) has been undertaken in dichloromethane solution with and without acetonitrile as a cosolvent. Spectroscopic monitoring of the reactions by UV-vis stopped-flow and NMR techniques over the temperature range -80 to +27 degrees C allowed the assessment of the effects of acid concentration, coordinating solvent (MeCN) concentration, temperature, and pressure. Protonation of (N-N)PtPh2 with HBF4 center dot Et2O in CH2Cl2/MeCN occurs with a kinetic preference for protonation at the metal, rather than at a phenyl ligand, and rapidly produces (N-N)PtPh2H(NCMe)(+) (Delta H-double dagger = 29 +/- 1 kJ mol(-1), Delta S-double dagger = -47 +/- 4 J K-1 mol(-1)). At higher temperatures, (N-N)PtPh2H(NCMe)(+) eliminates benzene to furnish (N-N)PtPh(NCMe)(+). This reaction proceeds by rate-limiting MeCN dissociation (Delta H-double dagger = 88 +/- 2 kJ mol(-1), Delta S-double dagger = +62 +/- 6 J K-1 mol(-1), Delta V-double dagger = +16 +/- 2 cm(3) mol(-1)). Protonation of (N-N)PtPh2 in dichloromethane in the absence of MeCN cleanly produces the Pt(II) pi-benzene complex (N-N)PtPh(eta(2)-C6H6)(+) at low temperatures. Addition of MeCN to a solution of the pi-benzene complex causes an associative substitution of benzene by acetonitrile, the kinetics of which were monitored by H-1 NMR (Delta H-double dagger = 39 +/- 2 kJ mol(-1), Delta S-double dagger = -126 +/- 11 J K-1 mol(-1)). When the stronger triflic acid is employed in dichloromethane/acetonitrile, a second protonation-induced reaction also occurs. Thus, (N-N)PtPh(NCMe)(+) produces (N-N)Pt(NCMe)(2)(2+) and benzene with no detectable intermediates (Delta H-double dagger = 69 +/- 1 kJ mol(-1), Delta S-double dagger = -43 +/- 3 J K-1 mol(-1)). The mechanisms for all steps are discussed in view of the accumulated data. Interestingly, the data allow a reinterpretation of a previous report on proton exchange between the phenyl and benzene ligands in (N-N)PtPh(eta(2)-C6H6)(+). It appears that the exchange occurs by a direct sigma-bond metathesis pathway, rather than by the oxidative cleavage/reductive coupling sequence that was proposed.