Abstract: A process and reagent kit for the determination of direct and total bilirubin in body fluids by coupling the bilirubin with a diazonium salt and determining the extinction change thereby brought about, by usual methods.The coupling is carried out in the presence of a buffer which adjusts the test solution to a weakly acidic to approximately neutral pH value preferably between pH 3.5 and 8. The diazonium salt is of the formula: ##STR1## in which R.sub.1 is a hydrogen or halogen atom or a lower alkyl, lower alkoxy or benzoylamino radical, R.sub.2 is a hydrogen atom or a lower alkyl or lower alkoxy radical, R.sub.3 is a hydrogen or halogen atom or a lower alkyl or lower alkoxy radical, whereby R.sub.1 and R.sub.3 do not simultaneously represent halogen atoms, and X is an anion forming a storage-stable salt with the diazonium cation.