A measurement of the B 0 decay parameters in the B 0 → J/ψϕ channel using an integrated luminosity of 14.3 fb−1 collected by the ATLAS detector from 8 TeV pp collisions at the LHC is presented. The measured parameters include the CP -violating phase ϕ s , the decay width Γ s and the width difference between the mass eigenstates ΔΓ s . The values measured for the physical parameters are statistically combined with those from 4.9 fb−1 of 7 TeV data, leading to the following: $$ \begin{array}{l}{\phi}_s=-0.090\pm 0.078\left(\mathrm{stat}.\right)\pm 0.041\left(\mathrm{syst}.\right)\mathrm{rad}\\ {}\varDelta {\varGamma}_s=0.085\pm 0.011\left(\mathrm{stat}.\right)\pm 0.007\left(\mathrm{syst}.\right){\mathrm{ps}}^{-1}\\ {}{\varGamma}_s=0.675\pm 0.003\left(\mathrm{stat}.\right)\pm 0.003\left(\mathrm{syst}.\right){\mathrm{ps}}^{-1}.\end{array} $$ In the analysis the parameter ΔΓ s is constrained to be positive. Results for ϕ s and ΔΓ s are also presented as 68% and 95% likelihood contours in the ϕ s -ΔΓ s plane. Also measured in this decay channel are the transversity amplitudes and corresponding strong phases. All measurements are in agreement with the Standard Model predictions.