The paper reports conditions suitable for recovering and processing 4-O-methylglucuronoxylans from birch and eucalyptus spent kraft cooking liquors. The black liquor originating from the initial heating-up phase in birch kraft cooking exhibits a high xylan and low lignin concentration. The maximum concentration of polymeric xylan was found in the cooking liquor just before the cook reached its final cooking temperature. For eucalyptus kraft cooking, however, lignin was more abundant than xylan in the cooking liquor throughout the whole cook. Birch and eucalyptus xylan, exhibiting good purity, were produced by employing a process chain involving, redrawing of xylan-rich spent liquor early in the cook, upgrading of the spent liquor employing ultrafiltration with diafiltration followed by precipitation and drying.