The CP-violating weak phase phi[s] of the Bs meson and the decay width difference Delta Gamma[s] of the Bs light and heavy mass eigenstates are measured with the CMS detector at the LHC using a data sample of Bs to J/Psi phi(1020) to mu+mu-K+K- decays. The analysed data set corresponds to an integrated luminosity of 19.7 inverse femtobarns collected in pp collisions at a centre-of-mass energy of 8 TeV. A total of 49,200 reconstructed Bs decays are used to extract the values of phi[s] and Delta Gamma[s] by performing a time-dependent and flavour-tagged angular analysis of the mu+mu-K+K- final state. The weak phase is measured to be phi[s] = -0.075 +/- 0.097 (stat) +/- 0.031 (syst) rad, and the decay width difference is Delta Gamma[s] = 0.095 +/- 0.013 (stat) +/- 0.007 (syst) inverse picoseconds.