We discovered pH-induced selective desorption and readsorption of weakly acidic polymers from polyacid or polybase layer-by-layer films. In situ attenuated total reflection Fourier transform infrared spectroscopy confirms the selectivity of polyacid release and shows that film response is caused by pH-induced charge imbalance. Experimentally the characteristic time of chain release tau scales with molar mass M as M(1.1+/-0.1). A new theoretical model of "sticky gel electrophoresis" of entangled polyacids agrees with experiments and predicts tau approximately MH, where H is film thickness. Neutron reflectivity shows that polyacid release results in disordering of the film structure.