The present invention provides a method for producing a film forming, fractionated novolak resin, by:a) condensing formaldehyde with one or more phenolic compounds, and thereby producing a novolak resin;b) adding a photoresist solvent, and optionally a water-soluble organic polar solvent;c) feeding the mixture into a liquid/liquid centrifuge and feeding a C.sub.5 -C.sub.8 alkane, water or aromatic hydrocarbon solvent into the liquid/liquid centrifuge at a ratio of optional water-soluble organic polar solvent and photoresist solvent to C.sub.5 -C.sub.8 alkane, water or aromatic solvent, of from 5:1 to 0.5:1;d) rotating the liquid/liquid centrifuge containing the mixture at a speed of at least 500 rpm and thereby separating the mixture into two phases, collecting the two phases;e) optionally separating the lighter phase (L) into two second phases;f) removing residual C.sub.5 -C.sub.8 alkane, water or aromatic hydrocarbon solvent from the heavier phase (H) from step d) and leaving the novolak resin dissolved in the photoresist solvent;A method is also provided for producing photoresist composition from such a fractionated novolak resin and for producing microelectronic devices using such a photoresist composition.