The ability of Xylaria species obtained from tropical wood and leaf litter to cause mass loss of lignin and carbohydrates in wood was examined in vitro with pure culture decomposition tests. The mass loss of wood of four tree species caused by nine Xylaria isolates ranged from 4.5% to 28.4% of the original wood mass. These Xylaria isolates have a potential ability to decompose lignin and other recalcitrant compounds, collectively registered as acid unhydrolyzable residue or Klason lignin, in wood. The origin of isolates (i.e., isolates from wood versus leaf litter) did not affect the mass loss of acid unhydrolyzable residue in wood. The Xylaria isolates tested generally caused selective decomposition of polymer carbohydrates in wood in preference to acid unhydrolyzable residue. Mass loss of acid unhydrolyzable residue caused by Xylaria isolates varied with tree species of wood and was negatively related to initial content of acid unhydrolyzable residue in wood, implying the limiting effect of lignin and recalcitrant compounds on wood decomposition by Xylaria isolates.