In order to generate sulfate radicals (SRs) as oxidizing species for the degradation of 2,4-dichlorophenol (2,4-DCP) in water, we explored heterogeneous activation of peroxymonosulfate (PMS) by supported cobalt catalysts. More attention was given to the effect of support materials (Al 2 O 3 , SiO 2 , TiO 2 ) and cobalt precursors (Co(NO 3 ) 2 , CoCl 2 , CoSO 4 ) on cobalt–support interaction, cobalt leaching, and reactivity of the catalysts. Especially, the feasibility of simultaneous generation of SRs and hydroxyl radicals (HRs) in PMS-Co/TiO 2 systems was first studied under ultraviolet (UV) radiation. Much lower cobalt leaching was observed in Co/Al 2 O 3 and Co/TiO 2 systems than that of Co/SiO 2 most probably due to their relatively strong cobalt-support interaction. Co/TiO 2 catalyst prepared with Co(NO 3 ) 2 , compared to CoCl 2 or CoSO 4 (where Cl − and SO 4 2− , respectively, were not completely removed upon heat treatment at 500°C), showed strong cobalt–support interaction, and thereby exhibited negligible cobalt leaching. Under UV radiation, Co/TiO 2 at Co/Ti molar ratio of 0.001 showed significant improvement in the degradation of 2,4-DCP due to HRs. The effective generation of HRs in the system can be explained with Co(III)-mediated charge transfer from the photoinduced electrons to PMS, inducing facilitation of photoinduced electron-hole separation. However, high cobalt loading (i.e., Co/Ti molar ratio of 0.1) on TiO 2 surface exhibited negligible enhancement of 2,4-DCP transformation under UV radiation since the penetration of UV light to TiO 2 was prohibited by the cobalt.