Acid chlorite delignification is frequently used to obtain a mixture of cellulose and hemicellulose known as holocellulose from biomass. While a majority of lignin is removed after holocellulose pulping, there appears to be a minor fraction of lignin that is more resistant to acid chlorite treatment and remains in the holocellulose even after repeated delignification treatment. This type of lignin, defined as hololignin, has not been characterized, is likely to contribute to biomass recalcitrance and is clearly of fundamental interest to understand its structural characteristics. In this study, hololignin isolated from poplar holocellulose was characterized with a wide array of techniques including GPC, quantitative <
sup>
13<
/sup>
C, DEPT-135, HSQC, and <
sup>
31<
/sup>
P NMR. The results were then compared to those from milled wood lignin (MWL), the representative native lignin isolated from poplar. NMR analysis demonstrated a depletion of cinnamyl aldehyde, acetyl group, and decrease of p-hydroxybenzoate structural units in hololignin. An enrichment of condensed structures in hololignin was observed. Hololignin also had a significantly lower molecular weight than MWL. Finally, hololignin is relatively enriched in guaiacyl units and has a lower S/G ratio, lower ?-O-4 ether linkages, fewer aliphatic and phenolic hydroxyl groups, and more carboxylic acid groups than MWL.