The specific volume of vitamin C has been investigated by X-ray powder diffraction as a function of temperature from 110¿K up to complete degradation around 440¿K. Its thermal expansion is relatively small in comparison with other organic compounds with an expansivity av of 1.2(3) × 10-4¿K-1. The structure consists of strongly bound molecules in the ac plane through a dense network of hydrogen bonds. The thermal expansion is anisotropic. Along the b axis, the expansion has most leeway and is about 10 times larger than in the other directions.