Abstract We study the semileptonic decays B→ X s e + e −, B→ X s μ + μ − in generic supersymmetric extensions of the Standard Model. SUSY effects are parameterized using the mass insertion approximation formalism and differences with the Constrained MSSM results are pointed out. Constraints on SUSY contributions coming from other processes (e.g. b→ sγ) are taken into account. Chargino and gluino contributions to photon and Z-mediated decays are computed and non-perturbative corrections are considered. We find that the integrated branching ratios and the asymmetries can be strongly modified. Moreover, the behavior of the differential forward–backward asymmetry remarkably changes with respect to the Standard Model expectation.