Kraft pulping of Douglas-fir(Pseudotsuga menziesii)was carried out from 90°C to a final temperature of 170°C at a heating rate of 1°C/min. At various stages of delignification, middle lamella and secondary wall enriched fractions were isolated from pulps and analyzed by nucleus exchange and alkaline nitrobenzene oxidation reactions to reveal the dissolution and condensation of lignin building units in these two morphological regions. For non-delignified Douglas-fir, the ratio of condensed/non-condensed phenyl nuclei varied significantly between middle lamella and secondary wall lignins. Values of 2.5/1 and 1/1 were found for middle lamella and secondary wall lignins, respectively. At early stages of pulping (below 110°C), the removal of phenyl nuclei in secondary wall lignin was due entirely to the dissolution of non-condensed phenyl nuclei. After the temperature reached 160°C, a rapid formation of diphenylmethane moieties in middle lamella was observed, whereas the formation of such moieties in secondary wall occurred after the peak pulping temperature (170°C) was reached.