Abstract The photochemical processes of Ni(II)bis(ethyl-3,5-di-t-butyl-4-hydroxybenzyl phosphonate) (I) in an inert solvent were investigated. The 2,6-di-t-butyl-4-methylphenol (II) and 3,3′,5,5′-tetra-t-butyl-4,4′-stilbenequinone (III) were found in the solution irradiated with light absorbed by (I). The quantum yield of the process for the decomposition of I is low ( Φ=1×10 −3 mol einstein −1). It was found that I quenches singlet oxygen in the anthracene +I+ hexane system irradiated with light at λ = 365 nm. The rate constant of the process was determined as equal to 2×10 8 dm 3 mol −1 s −1. It was found that the process of singlet oxygen deactivation by I consists mainly of the physical interaction between these two molecules.