The and B (0) mixing frequencies, Delta m (s) and Delta m (d) , are measured using a data sample corresponding to an integrated luminosity of 1.0 fb(-1) collected by the LHCb experiment in pp collisions at a centre of mass energy of 7 TeV during 2011. Around 1.8x10(6) candidate events are selected of the type (+ anything), where about half are from peaking and combinatorial backgrounds. To determine the B decay times, a correction is required for the momentum carried by missing particles, which is performed using a simulation-based statistical method. Associated production of muons or mesons allows us to tag the initial-state flavour and so to resolve oscillations due to mixing. We obtain < The hypothesis of no oscillations is rejected by the equivalent of 5.8 standard deviations for and 13.0 standard deviations for B (0). This is the first observation of mixing to be made using only semileptonic decays.