Abstract: In a process for the preparation of optionally-substituted 1,2,3,4-tetrahydro-9-cyanomethylcarbazol-1-ones of the formula ##STR1## wherein R denotes hydrogen, halogen, alkyl or alkoxy, by cyano-methylating optionally substituted 1,2,3,4-tetrahydrocarbazol-1-ones of the formula ##STR2## wherein R has the meanings mentioned above, the starting material of formula II is reacted, in a two-phase system consisting of water and a water-immiscible organic solvent, in the presence of a strong base and a known phase transfer catalyst, with a cyanomethylating agent of the formulaR.sup.1 --CH.sub.2 --CN (III),wherein R.sup.1 denotes halogen or a radical of the formula R.sup.2 --SO.sub.2 --O-- and R.sup.2 represents alkyl, phenyl or substituted phenyl.