Reported herein are thermochermical studies of hydrogen atom transfer (HAT) reactions involving transition metal H-atom donors (MLH)-L-II and oxyl radicals. [Fe-II(H(2)bip)(3)](2+), [Fe-II(H(2)bim)(3)](2+), (Co-II(H(2)bim)(3))(2+), and Ru-II(acac)(2)(py-imH) [H(2)bip = 2,2'-bi-1,4,5,6-tetrahydropyrimidine, H(2)bim = 2,2'-bi-imidazoline, acac = 2,4-pentandionato, py-imH = 2-(2'-pyridyl)imidazole)] each react with TEMPO (2,2,6,6-tetramethy-1-piperidinoxyl) or (Bu3PhO center dot)-Bu-t (2,4,6-tri-tert-butylphenoxyl) to give the deprotonated, oxidized metal complex (ML)-L-III and TEMPOH or (Bu3PhOH)-Bu-t. Solution equilibrium measurements for the reaction of [Co-II(H(2)bim)(2+) with TEMPO show a large, negative ground-state entropy for hydrogen atom transfer, -41 +/- 2 cal mol(-1) K-1. This is even more negative than the Delta S degrees(HAT) = -30 +/- 2 cal mol(-1) K-1 for the two iron complexes and the Delta S degrees(HAT) for Ru-II(acac)(2)(py-imH) + TEMPO, 4.9 +/- 1.1 cal mol(-1) K-1, as reported earlier. Calorimetric measurements quantitatively confirm the enthalpy of reaction for [Fe-II(H(2)bip)(3)](2+) + TEMPO, thus also confirming Delta S degrees(HAT). Calorimetry on TEMPOH + (Bu3PhO center dot)-Bu-t gives Delta H degrees(HAT) = -11.2 +/- 0.5 kcal mol(-1) which matches the enthalpy predicted from the difference in literature solution BDEs. A brief evaluation of the literature thermochemistry of TEMPOH and (Bu3PhOH)-Bu-t supports the common assumption that I Delta S degrees(HAT) approximate to 0 for HAT reactions of organic and small gas-phase molecules. However, this assumption does not hold for transition metal based HAT reactions. The trend in magnitude of I Delta S degrees I-HAT for reactions with TEMPO, Ru-II(acac)(2)(Py-imH) << (Fe-II(H(2)bip)(3)](2+) = [Fe-II(H(2)bim)(3)](2+) < [Co(II)bim)(3)](2+), is surprisingly well predicted by the trends for electron transfer half-reaction entropies, Delta S degrees(ET), in aprotic solvents. This is because both Delta S degrees(ET) and Delta S degrees(HAT) have substantial contributions from vibrational entropy, which varies significantly with the metal center involved. The close connection between Delta S degrees(HAT) and Delta S degrees(ET) provides an important link between these two fields and provides a starting point from which to predict which HAT systems will have important ground-state entropy effects.