With the aid of the Boltzmann superposition principle, the infinitesimal flow behaviour of a substance can be described in terms of a memory integral. The memory function in this integral can be expanded in a set of relaxation times and relaxation strengths: the relaxation spectrum. On basis of a specific form of the fluctuation-dissipation theorem, a physical meaning is proposed for the relaxation spectrum. The theory of complex functions enables one to show that the relaxation spectrum, determined from dynamic mechanical measurements, has a unique meaning. The infinitesimal flow behaviour, expressed in terms of the relaxation spectrum, is used to develop a general description of the phenomenon of thermoreversible gelation. This description method was applied to a styrene-isoprene-styrene triblock copolymer solution.