AbstractGel‐point methods for the determination of the functionality of some of the most widely used types of industrial polyisocyanate and polyisocyanate derivatives have been investigated. No single method was applicable to all types, and the most suitable method for each type was determined.With aliphatic polyisocyanates such as N,N′,N‐tris(6‐isocyanatohexyl)biuret I and II b or c (derived from trimethylolpropane and diisocyanates), the most satisfactory method was gel‐point titration with amino‐terminated polyethers. This method was unsuitable for the more reactive aromatic polyisocyanates IIa (from trimethylolpropane and tolylene diisocyanate) or III (poly(methylene phenylene) polyisocyanate), but these could be gelled by reaction with water. Gel‐point condensations with polyamines could be used for the less reactive phenol‐ and ketoxime‐blocked polyisocyanates.Poly(carbamoyl sulfonate) (VI) could be gelled either by hydrolysis to a polyurea in the presence of excess tertiary amine or by reaction with a diamine in the presence of a slight excess of tertiary amine to neutralize the bisulfite released. In all the gelations studied, the functionalities depended on concentration as a result of intramolecular reactions; this is a possible source of error in functionality determinations, particularly in the