Three types of kraft pulps (unbleached softwood. unbleached hardwood and oxygen prebleached hardwood kraft pulps) were chlorinated at various levels of chlorine ratio (0.1-0.3) and residual chloroorganics in pulps were extracted in high yields by aqueous dioxane. Neutral sugar contents of these extracts were in the range of 1 to 5%. Extracted chloroorganics were treated by aqueous alkali in a similar condition as the E 1-stage of bleaching. Structural features of extracted chloroorganics and their alkali-treated products were analyzed by' H-NMR spectroscopy. FT-IR spectroscopy and others. These results revealed that the structures of chloroorganics were quite different from that of native lignin. Only few aromatic protons are observed in' H-NMR spectra. Instead of this, chloroorganics are very rich in methyl, methylene and methine structures and carboxylic acids. Model experiments using milled wood lignin (MWL) revealed that these structural features can be reasonably explained by the structural modification, especially by an oxidative ring opening, of residual lignins in unbleached kraft pulps during chlorine treatment.