A measurement of Bs J/psi phi decay parameters, including the CP-violating weak phase phi_s and the decay width difference Delta Gamma_s is reported, using 4.9 fb-1 of integrated luminosity collected in 2011 by the ATLAS detector from LHC pp collisions at a centre- of-mass energy sqrt(s) = 7 TeV. The mean decay width Gamma_s and the transversity amplitudes |A0(0)|^2 and |A||(0)|^2 are also measured. The values reported for these parameters are: phi_s = 0.22 \pm 0.41 (stat.) \pm 0.10 (syst.) rad Delta Gamma_s = 0.053 \pm 0.021 (stat.) \pm 0.010 (syst.) ps-1 Gamma_s = 0.677 \pm 0.007 (stat.) \pm 0.004 (syst.) ps-1 |A0(0)|^2 = 0.528 \pm 0.006 (stat.) \pm 0.009 (syst.) |A||(0)|^2 = 0.220 \pm 0.008 (stat.) \pm 0.007 (syst.) where the values quoted for phi_s and Delta Gamma_s correspond to the solution compatible with the external measurements to which the strong phase delta perpendicular is constrained and where Delta Gamma_s is constrained to be positive. The fraction of S-wave KK or f0 contamination through the decays Bs -> J/psi K+K-(f0) is measured as well and is found to be consistent with zero. Results for phi_s and Delta Gamma_s are also presented as 68%, 90% and 95% likelihood contours, which show agreement with Standard Model expectations.