A study of b semileptonic decays into D, Dπ ± and D ∗π ± final states is presented. The D 0, D + and D ∗+ mesons are exclusively reconstructed in Z decay data recorded from 1992 to 1995 in the DELPHI experiment at LEP. The overall branching fractions are measured to be: BR(b→D 0ℓ − ν ̄ ℓX)=(7.04±0.34 (stat)±0.36 (syst.exp)±0.17 ( BR D))% , BR(b→D +ℓ − ν ̄ ℓX)=(2.72±0.19 (stat)±0.16 (syst.exp)±0.18 ( BR D))% , BR(b→D ∗+ℓ − ν ̄ ℓX)=(2.75±0.17 (stat)±0.13 (syst.exp)±0.09 ( BR D))% , where the D 0 and D + results include also contributions from D ∗0 and D ∗+ decays. A fit to the distribution of the π ± impact parameter to the primary interaction vertex provides a measurement of the b semileptonic branching fractions into the D 0 π ± X, D + π ± X and D ∗+π ±X final states. Assuming that single pion decay modes of B mesons dominate, the partial rates for B ̄ →Dπℓ − ν ℓ ̄ and B ̄ →D ∗πℓ − ν ℓ ̄ have been obtained, corresponding to a total branching fraction: BR( B ̄ →Dπℓ − ν ℓ ̄ )+ BR( B ̄ →D ∗πℓ − ν ℓ ̄ )=(3.40±0.52 (stat)±0.32 (syst))% . This result agrees well with the observed difference between the total B semileptonic branching fraction and the sum of the B ̄ →Dℓ − ν ℓ ̄ and D ∗ℓ − ν ℓ ̄ branching fractions.