The synthesis of lignin-phenol-formaldehyde (LPF) was studied to determine its optimum operating conditions. The lignin proposed as phenol substitute has been the softwood ammonium lignosulfonate. The resin synthesis was optimized by varying the methylolated lignosulfonate content, 18-52%; the sodium hydroxide to phenol-modified lignosulfonate molar ratio, 0.3-0.94; and the formaldehyde to phenol-modified lignosulfonate molar ratio, 1.1-3.5. The parameters employed in the characterization of LPF resins were free phenol, free formaldehyde, gel time, alkaline number, viscosity, pH, solid content, and chemical structure changes. The properties of LPF resin comply with the requirements for its utilization in plywood manufacture. (C) 2004 Wiley Periodicals, Inc.