The first measurement of the cross section ratio σtt¯bb¯/σtt¯jj is presented using a data sample corresponding to an integrated luminosity of 19.6 fb −1 collected in pp collisions at s=8 TeV with the CMS detector at the LHC. Events with two leptons (e or μ ) and four reconstructed jets, including two identified as b quark jets, in the final state are selected. The ratio is determined for a minimum jet transverse momentum pT of both 20 and 40 GeV/ c . The measured ratio is 0.022±0.003(stat)±0.005(syst) for pT>20 GeV/c . The absolute cross sections σtt¯bb¯ and σtt¯jj are also measured. The measured ratio for pT>40 GeV/c is compatible with a theoretical quantum chromodynamics calculation at next-to-leading order.