A nanocrystalline powder of the waylandite-structured bismuth hydroaluminophosphate was obtained under hydrothermal conditions at 200 • C, 7 MPa and pH 7, and characterized by X-ray diffractometry, scanning electron microscopy (SEM), and energy dispersive microanalysis (EDAX). The simultaneous thermal analysis and high-temperature X-ray diffractometry have shown that the crystal-chemical formula of this compound can be represented as BiAl 3 (PO 4 ) 2 O(OH) 4 •(H 2 O). This compound retains its structure and crystallite size (∼65 nm) up to about 500 • C. It has been determined that the decomposition of this compound in the 540-800 • C range results in the formation of Bi 2 O 3 , Bi 2 Al 4 O 9 and AlPO 4 phases. At temperatures above 800 • C, a complete thermal decomposition of Bi 2 Al 4 O 9 and the formation of crystalline α-Al 2 O 3 occur in this system, while Bi 2 O 3 keeps evaporating during the isothermal exposure. KEYWORDS nanocrystals, waylandite-structured, BiAl 3 (PO 4 ) 2 O(OH) 4 •(H 2 O), thermal stability, hydrothermal synthesis, nature-like technologies. ACKNOWLEDGEMENTS X-ray diffraction studies and elemental analyses of samples were performed employing the equipment of the Engineering Center of the