A measurement of the B s 0 decay parameters in the B s 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: ϕ s = − 0.090 ± 0.078 stat . ± 0.041 syst . rad Δ Γ s = 0.085 ± 0.011 stat . ± 0.007 syst . ps − 1 Γ s = 0.675 ± 0.003 stat . ± 0.003 syst . ps − 1 . $$ \begin{array}{l}{\phi}_s=-0.090\pm 0.078\left(\mathrm{stat}.\right)\pm 0.041\left(\mathrm{syst}.\right)\mathrm{rad}\\ {}\varDelta {\varGamma}_s=0.085\pm 0.011\left(\mathrm{stat}.\right)\pm 0.007\left(\mathrm{syst}.\right){\mathrm{ps}}^{-1}\\ {}{\varGamma}_s=0.675\pm 0.003\left(\mathrm{stat}.\right)\pm 0.003\left(\mathrm{syst}.\right){\mathrm{ps}}^{-1}.\end{array} $$