This study focuses on the possibility of cleaning of industrial wastewater with catalytically active magnetic nanoparticles. Cobalt ferrite synthesized by the co-precipitation method was used, as prepared or after surface modification with a silica precursor. Electronic absorption spectra were recorded and analyzed to obtain the phenol degrading rate for various experimental design variants. Treating with pristine magnetic nanoparticles under simultaneous exposure to ultraviolet radiation resulted in similar degrading rates for 4 g/L and 8 g/L pristine nanoparticles, while, for silanized nanoparticles, the degrading rates were slightly increased. Along with ultraviolet irradiation and magnetic nanoparticles, hydrogen peroxide was also added, which led to significant enhancement of phenol degradation, for both pristine and silanized nanoparticles. It is proposed that photo-Fenton processes, triggered by metal ions at the nanoparticle surface and water photolysis and sustained by hydrogen peroxide decomposition, occurred to gradually decompose phenol to simpler compounds.