The effect of low-frequency ultrasound on the treatment of birch sawdust with hot water and diluted acid was studied for the first time under high pressure and temperature. When treated with diluted formic acid solution, the production of furfural dramatically increased, emphasizing the straightforward hydrolysis of hemicellulose. The highest furfural yield (43–44 %) was achieved with 5 % formic acid. The most significant decrease in crystallinity index (from 56 % to 50 %) compared to treatment without ultrasound was observed with 2.5 % formic acid, using a 70:30 pulse mode with 70 % amplitude of the nominal power deliverable by the generator and an additional pressure of 3 bar. In diluted acid experiments, the remaining solid fraction, which presumably contained mainly cellulose and lignin, exhibited a completely different shape compared to the starting material, resulting in a kind of “wooden paste” with reduced particle size. The impact of additional pressure appeared to be significant, and this is further discussed in relation to the determination of the acoustic power under such rigorous experimental conditions. Based on the results obtained, ultrasound combined with hot water / diluted acid treatment shows potential for the efficient utilization of birch sawdust for platform chemicals.