The oxidation of benzothiophene (BT), dibenzothiophene (DBT), and 4,6-dimethyldibenzothiophene (DMDBT) by H
2O
2 to the corresponding sulfoxides and sulfones has been studied under homogeneous conditions in MeCN with the compounds [Cp*
2M
2O
5] (M = Mo (
1), W (
2)) as precatalysts. The W system is ca
. 100 times more efficient than the Mo analogue, while the relative reactivity of the thiophene substrates is approximately DBT/DMDBT/BT ≈ 10/5/1. For all reactions rate constants for both steps (thiophene derivative to sulfoxide,
k1; sulfoxide to sulfone,
k2) were measured. While
k1 ≈
k2 for DBT and DMDBT,
k1 <<
k2 for BT, independent of catalyst. Activation parameters for the stepwise oxidations of thiophene derivative to sulfoxide (BT to BTO, Δ
H = 11.4(5) kcal mol
−1 and Δ
S = –26.1(1.6) eu; DBT to DBTO, Δ
H = 7.7(6) kcal mol
−1 and Δ
S = –33(2) eu) and sulfoxide to sulfone (BTO to BTO
2, Δ
H = 10.8(5) kcal mol
−1 and Δ
S = –21.8(1.6) eu; DBTO to DBTO
2, Δ
H = 10.3(9) kcal mol
−1 and Δ
S = –25(3) eu) were calculated from variable temperature studies using [Cp*
2W
2O
5]. DFT calculations suggest that the greater reactivity of DBT relative to BT is not caused by ground-state effects but rather by a transition-state effect associated with the greater thermodynamic gain in DBT oxidation.