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 5 - c 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 5 - c 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 into two second phases ; f ) removing residual c 5 - c 8 alkane , water or aromatic hydrocarbon solvent from the heavier phase 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 .