The thermal decomposition mechanism of synthetic Al(OH)(3) (gibbsite) was studied in situ by neutron thermodiffractometry in an ambient atmosphere from room temperature to 600 degrees C with 50 degrees C steps. Gibbsite decomposed to yield AlO.(OH) (boehmite) and then poorly crystallized chi-Al2O3. Rietveld analysis was used to refine the cell parameters' variation of gibbsite and its thermal expansion coefficients were obtained: for the a-axis: 15 +/- 1 x 10(-6) K-1, for b: 10 +/- 2 x 10(-6) K-1, and for c: 17 +/- 2 x 10(-6) K-1.