We present the effect of molecular weight (MW) of polyelectrolytes (PEs) on the disintegration behavior of weak PE multilayer films consisting of linear poly(ethylene imine) (LPEI) and poly(methacrylic acid) (PMAA). The multilayer films prepared by the spin-assisted layer-by-layer deposition have well-ordered internal structures and also show the linear thickness growth behavior regardless of MWs of PMAA. The well-defined weak PE multilayer films were subject to disintegration into bulk solution when the electrostatic interactions between LPEI and PMAA layers were reduced by treatment at pH 2. However, we demonstrated the change in the disintegration mode and kinetics (i.e., from burst erosion to controlled surface erosion) as a function of MW of PMAA based on neutron reflectivity and quartz crystal microbalance with dissipation, revealing the correlation between the structural changes and the viscoelastic responses of the weak PE films upon pH treatment. Also, the unique swelling behavior as well as the significant increase in dissipation energy was monitored before the complete disintegration of the multilayer films containing high MW PMAA, which is believed to originate from their slow rearrangement kinetics within the film. We believe that the results shown in this study provide chain-level understanding as to the MW-dependence on pH-triggered disintegration mechanism of weak PE multilayer films.