We discuss the importance of inelasticity in the P -wave ππ amplitude on the Dalitz distribution of 3π events in J/ψ decay. The inelasticity, which becomes sizable for ππ masses above 1.4 GeV, is attributed to KK → ππ re-scattering. We construct an analytical model for the two-channel scattering amplitude and use it to solve the dispersion relation for the isobar amplitudes that parametrize the J/ψ decay. We present comparisons between theoretical predictions for the Dalitz distribution of 3π events with available experimental data.Here is the polarization vector of the J/ψ, the Dalitz invariants are defined by s ij = (p i + p j ) 2 for i, j = ±, 0 and satisfy s +− + s 0+ + s −0 = M 2 + 3m 2 π ,p i = p i /|p i |, and the scalar form factor F describes the dynamics of