This study presents the effects of immersion depth and duration on the chemical composition of Erythrophleum fordii Olive and Vatica tonkinensis A. Chev. wood. The results showed that both immersion depth and duration significantly influenced the chemical degradation and structural changes of the wood. The contents of water-soluble extractives in both cold and hot water extraction decreased sharply, especially after the first 12 months of immersion at 30 cm depth, reflecting the leaching of hydrolysable compounds. Extractives soluble in organic solvents were less affected but still showed a gradual reduction over time. Total extractive content in Erythrophleum fordii wood decreased by 44.9%, and in Vatica tonkinensis wood by 40.4% after 24 months at 30 cm depth. At 90 cm depth, the decrease was only 32.2% and 26.2%, respectively. Lignin content tended to increase (35.1 - 35.9%) due to the relative reduction of cellulose and hemicellulose. Both cellulose and hemicellulose content decreased, with more pronounced losses at 30 cm depth, likely due to increased microbial and enzymatic activity in oxygen-exposed zones. Maximum water content increased with immersion time and decreased with depth, reaching the highest levels in 30 cm samples after 24 months (an increase of 11.49% for Erythrophleum fordii and 25.97% for Vatica tonkinensis). This indicates cell wall degradation and increased porosity, enhancing water retention. To slow down biological degradation, samples should be submerged at a depth of at least 60 cm below the water surface in the immersion tank.