The time-dependent C P asymmetry in B 0 s → J / ψ K + K − decays is measured using p p collision data at √ s = 7     TeV , corresponding to an integrated luminosity of 1.0     fb − 1 , collected with the LHCb detector. The decay-time distribution is characterized by the decay widths Γ L and Γ H of the light and heavy mass eigenstates of the B 0 s − ¯¯¯ B 0 s system and by a C P -violating phase ϕ s . In a sample of 27 617 B 0 s → J / ψ K + K − decays, where the dominant contribution comes from B 0 s → J / ψ ϕ decays, these parameters are measured to be ϕ s = 0.07 ± 0.09 ( stat ) ± 0.01 ( syst )     rad , Γ s ≡ ( Γ L + Γ H ) / 2 = 0.663 ± 0.005 ( stat ) ± 0.006 ( syst )     ps − 1 , and Δ Γ s ≡ Γ L − Γ H = 0.100 ± 0.016 ( stat ) ± 0.003 ( syst )     ps − 1 , corresponding to the single most precise determination of ϕ s , Δ Γ s , and Γ s . The result of performing a combined analysis with B 0 s → J / ψ π + π − decays gives ϕ s = 0.01 ± 0.07 ( stat ) ± 0.01 ( syst )     rad , Γ s = 0.661 ± 0.004 ( stat ) ± 0.006 ( syst )     ps − 1 , and Δ Γ s = 0.106 ± 0.011 ( stat ) ± 0.007 ( syst )     ps − 1 . All measurements are in agreement with the Standard Model predictions.