The present invention provides a process for the production of intermediate compounds of formula (I), wherein the substituents are as defined herein. The process comprises reacting a substituted aniline with aqueous HX, followed by removal of water by azeotropic distillation and diazotization and pyrolysis with an organic nitrite at elevated temperatures in the absence of a copper catalyst. Alternatively, gaseous HX can be used to substitute aqueous HX in the process. Consequently, a step of water removal by azeotropic distillation can be eliminated. The intermediate compounds of formula I are suitable as intermediates in the preparation of herbicidally active 3-hydroxy-4-aryl-5-oxopyrazoline derivatives.