The measurement of the relative width difference ∆Γd/Γd of the B0–B¯0 system using 25.2 fb−1 of integrated luminosity collected by the ATLAS detector at the LHC in pp collisions at √s=7 TeV and √s=8 TeV is presented. The analysis described in this thesis incorporates results previously published by the ATLAS collaboration while providing greater detail and additional studies. The measured value is ∆Γd/Γd = (−0.1±1.1 (stat.)±0.9 (syst.))×10−2 . Currently, this is the most precise single measurement of ∆Γd/Γd. It agrees with the Standard Model prediction and measurements by other experiments.