Intercalation of aromatic compounds into DNA can be appreciated by using, in a complementary way, fibre X-ray diffraction and measurement of variations of the fibre dimensions. The different behaviours resulting from an actual intercalation or an association of small molecules into the DNA grooves can be distinguished by this experimental method which provides significant results for proflavine, ethidium bromide, a platinum compound, 9-aminoacridine and ellipticine.