The responses and charge-state distributions of poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) (Mn = 400–1000 g/mol) at constant electrolyte concentrations (NaCl, 10−3 mol/L) in electrospray were studied as a function of the analyte concentration in the concentration range of 10−2 to 10−7 mol/L. Single charging occurred in the case of PEG and PPG with Mn = 400 g/mol. It was observed that the response changed nearly linearly with the concentration of the polymer in the low concentration region but above a certain concentration the response leveled off. Saturation of the response at higher concentrations of PEG and PPG of Mn = 1000 g/mol, where double and triple charging occurred, was also observed. Based on the equilibrium partitioning theory a model was developed to account for the response curves and charge-state distributions of singly, doubly and triply charged adduct ions generated from solutions of PEG and PPG. The experimental and the theoretical response curves and charge-state distributions calculated with the model are in fairly good agreement. Copyright © 2006 John Wiley & Sons, Ltd.