The most recent results on mixing and CP violation in the B0 and B0s systems published by the ATLAS experiment at the LHC are presented. The measurement of the B0s → J/ψφ decay parameters, using data collected in pp collisions at √s = 7 TeV and √s = 8 TeV and corresponding to 4.9 fb−1 and 14.3 fb−1 of integrated luminosity, includes the CP-violating phase φs and the width difference of the mass eigenstates ∆Γs . The combined Run-1 results are φs = −0.098±0.084(stat.)±0.040(syst.) rad and ∆Γs = 0.083±0.011(stat.)±0.007(syst.) ps−1 . The measured values agree with the Standard Model predictions. The second result presented is the measurement of the relative width difference ∆Γd/Γd of the B 0–B¯0 system, using data collected in pp collisions at √s = 7 TeV and √s = 8 TeV, and corresponding to a total integrated luminosity of 25.2 fb−1 . 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.