We report on the measurement of W-boson pair-production with the L3 detector at LEP at an average centre-of-mass energy of 172.13 GeV. In a data sample corresponding to a total luminosity of 10.25 pb⁻¹ we select 110 four-fermion events with pairs of hadronic jets or leptons with high invariant masses. Branching fractions of W decays into different fermion-antifermion pairs are determined with and without the assumption of charged-current lepton universality. The branching fraction for hadronic W decays is measured to be: B(W → hadrons) = 64.2^+3.7_−3.8 (stat.) ± 0.5 (syst.) %. Combining all final states the total cross section for W-pair production is measured to be: σ_WW = 12.27^+1.41_−1.32 (stat.) ± 0.23 (syst.) pb. The results are in good agreement with the Standard Model.Facultad de Ciencias Exactas