Douglas fir <i>(Pseudotsuga menziesii) treated with ammonium dihydrogen orthophosphate was pyrolyzed under vacuum at temperatures from 250 to 600C and for times from 5 to 240 minutes, and the electron spin resonance (ESR) spectra of the residues were analyzed. The percentage mass loss, the line width and the g-factor were smaller; the unpaired spin concentration, in general, was larger than the same spectral parameters for residues of the untreated specimens pyrolyzed under the same conditions as the treated specimens. Evaluation of the spectral parameters indicated that the flame retardant acted as a deoxygenation agent in the pyrolysis of wood. -- AATA