[Ru(tpy)(picSO)]BF4 (tpy: 2,2':6',2 ''-terpyridine, picSO: 6-[(methylsulfinyl)methyl]picolinate) undergoes profound linkage isomerization behavior between stable S-bound and metastable O-bound isomers. S-[Ru-II(tpy)(picSO)]BF4 experiences photoisomerization to O-[Ru-II(tpy)(picSO)]BF4 upon irradiation with 436 nm light, whereas the back reaction is triggered with 546 nm light. Electrochemical linkage isomerization also takes place.