The semileptonic branching fractions for primary and cascade b decays {BR}(brightarrow ell^-), BR(brightarrow crightarrow ell^+) and BR(brightarrowbar crightarrow ell^- were measured in hadronic Z decays collected by the DELPHI experiment at LEP. The sample was enriched in b decays using the lifetime information and various techniques were used to separate leptons from direct or cascade b decays. By fitting the momentum spectra of di-leptons in opposite jets, the average b mixing parameter bar{χ} was also extracted. The following results have been obtained: begin{eqnarray*} BR({brightarrow ell^- ell^-}) &=& (10.70±0.08 (stat)±0.21(syst)_{+0.44}^{-0.30}(model))% BR({brightarrow crightarrow ell^+})&=& (7.98±0.22 (stat)±0.21(syst)^{+0.14}_{-0.20}(model))% BR({brightarrow bar c rightarrow ell^-})&=& (1.61±0.20 (stat)±0.17(syst)^{+0.30}_{-0.44}(model))% bar{χ} &=& 0.127±0.013 (stat)±0.005(syst)±0.004(model) end{eqnarray*}