The oxygen molecule in its ground triplet state (3O2) is a strong electron acceptor. Electron transfer to 3O2 to form a superoxide anion is an important elementary step in many chemical and biological processes. If this transfer occurs from a spin 1/2 paramagnetic particle where the total spin of the reactants is equal to 3/2, the reaction is spin-forbidden. In liquids, the significant dipole-dipole electron spin interaction in 3O2 is supposed to mix the non-reactive quartet and reactive doublet states at a time scale of ∼10 ps, thus avoiding the barrier. To elucidate the role of spin effects in the electron transfer to 3O2, we studied this reaction over a range of more than three orders of magnitude of the relative diffusion coefficient (D) of the reactants. It was found that spin effects during electron transfer to 3O2 become insignificant when D < 10-9 m2 s-1. In the range of intermediate D values (10-9 m2 s-1 < D < 10-8 m2 s-1) - which corresponds to some reactions of oxygen with small radicals in aqueous solutions - the effective spin factor decreases with increasing D value. If D > 10-8 m2 s-1, the electron transfer is spin-selective with the spin factor of 1/3 as determined by the spin statistics. At such D values, the reaction encounter time may exceed the expected quartet-doublet mixing time by almost an order of magnitude. The reduced rate of quartet-doublet transitions within the encounter complex in the reaction with 3O2 has been explained by the spin-exchange interaction and chemical Zeno effect.