Cobalt salen-type [salen = N,N'-bis(salicylidene)ethylenediamine] complexes 1-6 were studied as catalysts for dioxygen activation in the oxidation of veratryl alcohol in basic aqueous conditions. The complexes Co(salen) (1), Co(alphaCH(3)salen) (2) [alphaCH(3)salen = N,N'-bis(alphamethylsalicylidene)ethylenediamine], Co(4OHsalen) (3) [4OHsalen = N,N'-bis(4-hydroxosalicylidene)ethylenediamine], Co(sulfosalen) (4) [sulfosalen = N,N'-bis(5-sulfonatosalicylidene)ethylenediamine], Co(acacen) (5) [acacen = N,N'-bis(acetylacetone)ethylenediamine] and Co(N-Me-salpr) (6) [N-Me-salpr = bis(salicylideniminato-3-propyl)methylamine] were chosen to examine the influence of ligand structure on the catalytic activity. The effect of reaction conditions on the oxidation of veratryl alcohol was studied by varying temperature, pH, time or the nature and amount of the axial base needed to enhance the activity of complexes 1-5. The catalytic behaviour of the studied complexes was shown to be very depended on the applied conditions and distinct differences could be observed among the complexes. In all reactions, veratraldehyde was the only product observed. The unsubstituted complex 1 was the most efficient catalyst in the studied system achieving turnover numbers of up to 28 at 80degreesC and pH 12.5.