Design of experiment (DoE) is applied to establish the optimum ionization conditions for analyzing synthetic polymers via coupled size exclusion chromatography electrospray ionization mass spectrometry (SEC-ESI-MS) yielding maximum ionization efficiency. The ion source conditions were optimized with regard to the ionization efficiency, the amount of fragmentation, as well as the formation of salt adducts. A D-optimal experimental design was employed for this purpose and the recorded data were evaluated by a quadratic response surface model, accounting for possible interactions between the individual source settings. It was established that the ionization efficiency can be improved by up to one order of magnitude without compromising the softness of the ionization process and that optimal ionization conditions are found at similar source settings regardless of the charge state. The present optimization exercise therefore provides a hands-on guide for the use of experimental design to determine optimum ionization conditions during the SEC-ESI-MS of functional polymers.