The specific surface areas of various particle size ranges of wood and peat were determined. The methods employed are mercury porosimetry, nitrogen adsorption, and solution isotherms. The results indicate that dye solution isotherms offer an accurate means of surface area measurement, however, the values obtained are dependent on (a) the chemical nature of the solute and adsorbent and (b) the molecular dimensions of the solute. Nitrogen isotherms indicate specific surfaces of (21 × 103)−(27 × 103) m2 kg−1 for wood depending on the particle size, while an area of 26.5 × 103 m2 kg−1 was obtained for peat independent of particle size. Acid dye isotherms yield specific surfaces considerably lower than nitrogen isotherm values; for wood (7.3 × 103)−(9.6 × 103) m2 kg−1 and for peat, (5.2 × 103)−(11.8 × 103) m2 kg−1. Basic dye studies, using wood, indicate surface areas similar in magnitude to those obtained from nitrogen isotherms. For peat, however, very large apparent surface areas are obtained (∼100 × 103 m2 kg−1) and are attributed to chemical interaction between dye molecules and adsorbent and stacking of dye molecules.