The complexes cis- and trans-[Pd(C6F5)2(tht)2] (tht = tetrahydrothiophene) spontaneously isomerize in chloroform to a cis-trans equilibrium mixture, where the cis isomer is the predominant species. The first-order forward rate constant, k(tc), and the equilibrium constant, K(eq), have been measured at different temperatures by proton NMR. The isomerization suffers mass-law retardation by added tht and is characterized by a high value of the enthalpy of activation (DELTAH(tc)double dagger = 137 +/- 6 kJ mol-1) and a large positive value of the entropy of activation (DELTAS(tc)double dagger = 83 +/- 19 J K-1 mol-1). In contrast, the substitution of tht by 2-methylpyridine in trans-[Pd(C6F5)2(tht)2] is characterized by a low enthalpy of activation (DELTAH(N)double dagger = 51 +/- 2 kJ mol-1) and a negative entropy of activation (DELTAS(N)double dagger = -114 +/-4 J K-1 mol-1). These findings are consistent with the usual associative mode of activation for the substitution reaction, while for the isomerization a mechanism is suggested involving the dissociative loss of tht and the interconversion of two geometrically distinct three-coordinate intermediates.