Autocatalyzed isopropanol-water extraction is a potentially high-value lignin isolation methodology. A solution of isopropanol-water (2-PrOH/ H2O, 7:3, v/v) was used on willow powder (Salix matsudana cv. Zhuliu), at varying temperatures (160 °C to 210 °C) and reaction times (6 h to 12 h). The highest yield (61 wt%) and purity (93.7%) with low weight/number average molecular weight (1681/1061 g/mol) was observed at a treatment temperature and time of 190 °C and 10 h, respectively. Fourier-transform infrared (FTIR) and two-dimensional nuclear magnetic resonance (2D-NMR) spectroscopy indicated that the lignin was composed of syringyl (S) and guaiacyl (G) units that were not severely damaged. More specifically, 31P-NMR showed that S units were dominant. Finally, a majority of glucan was preserved.