This invention relates to isoindolone derivatives of general formula (I) in which the radicals R are hydrogen atoms or together form a bond, the symbols R' are phenyl radicals which can be substituted by a halogen atom or a methyl radical in position 2 or 3, X is an oxygen or sulphur atom or a radical N--R.sub.3, R.sub.3 being H, optionally substituted alkyl or dialkylamino, R.sub.1 is optionally substituted phenyl or cyclohexadienyl, naphthyl or a heterocycle and R.sub.2 is H, halogen, OH, alkyl, amionoalkyl, alkylaminoalkyl, dialkylaminoalkyl, alkoxy, alkylthio, acyloxy, carboxyl, optionally substituted alkoxycarbonyl, benzyloxycarbonyl, amino or acylamino, in their (3aR,7aR) forms or in the (3aRS,7aRS) form or their mixtures, and if appropriate their salts where such exist, and their preparation.The new derivatives according to the invention are particularly valuable as antagonists of substance P. ##STR1##