The octamolybdate salt (H(3)biim)(4)[beta-Mo8O26] (1) has been prepared in good yield by hydrolysis of the complex [MoO2Cl2(H(2)biim)] (H(2)biim = 2,2'-biimidazole). Compound 1 showed a good performance as a (pre)catalyst for the epoxidation of olefins using either tert-butylhydroperoxide (TBHP) or hydrogen peroxide as oxidant. With the ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide) as co-solvent and TBHP as oxidant, the catalyst system could be reused several times without loss of activity for the epoxidation of the bio-olefin methyl oleate (Ole). Compound 1 is the first polyoxomolybdate used for the Ole/TBHP reaction in IL medium.