We report on average crystallite size-dependent structural, spectroscopic and thermoanalytic studies of Bi2Fe4O9 synthesized by a sol-gel method. In-situ heating X-ray diffraction revealed transformation of an X-ray amorphous precursor into a rhombohedral perovskite-type BiFeO3 followed by a second transformation into orthorhombic mullite-type Bi2Fe4O9 phase. Twelve samples with average crystallite sizes between 35.3(4) and 401(17) nm were produced using calcination of the precursor for 2 h at temperatures between 900 and 1073 K. The average crystallite sizes calculated from X-ray diffraction and transmission electron microscopy for selective samples are in excellent agreement. X-ray powder diffraction data Rietveld refinements demonstrate each structural feature as a function of the average crystallite size. Both in the in-situ and ex-situ experiments the metric parameters evolve through expansion of a- and contractions of b- and c-cell parameters, leading to a parabolic expansion of the cell volume. The...