Treatment of (meso-tetraphenylporphyrinato)tin(IV) sulfide, (TPP)Sn=S, with (meso-tetra-p-tolylporphyrinato)tin(II), (TTP)Sn-II, in toluene results in the reversible exchange (K = 1.21 +/- 0.03 at -10 degrees C) of a sulfur ligand to form (TPP)Sn-II and (TTP)Sn=S. The net result is a formal two-electron redox process between Sn-II and Sn-IV. This occurs with a second-order rate constant at 30 degrees C of 0.40 +/- 0.05 M(-1) s(-1) (Delta H-double dagger = 10.9 +/- 0.9 kcal/mol, Delta.S-double dagger = -24.1 +/- 2.8 cal(mol K)(-1)). Similarly, treatment of (meso-tetraphenylporphyrinato)tin(IV) selenide, (TPP)Sn=Se, with (meso-tetra-p-tolylporphyrinato)tin(II), (TTP)Sn-II, in toluene results in the reversible exchange (K = 1.45 +/- 0.13 at -10 degrees C) of a selenium ligand to form (TPP)Sn-II and (TTP)Sn=Se. This reaction occurs with a second-order rate constant at 30 degrees C of 87.3 +/- 8.06 M(-1) s(-1) (Delta H-double dagger 9.3 +/- 0.5 kcal/mol, Delta S-double dagger = -18.8 +/- 1.5 cal(mol.K)(-1)). Discussion of an inner sphere mechanism involving a mu-sulfido or a mu-selenido bridged intermediate is presented. The rate ratio of selenium to sulfur atom transfer is 218:1 at 30 degrees C. This rate behavior follows the "normal" trend as observed for the analogous halogen transfer reactions (I- > Br- > Cl- > F-).