To perform on-line monitoring of the absolute weight-averaged mass, M-W, of the polymers produced in a polymerization reaction, refractometer (RI), ultraviolet absorbance (UV), and time-dependent static light scattering (TDSLS) detectors were placed in series, and a diluted stream of reactant solution was made to flow through them. The technique allows rapid determination of time-dependent reaction "signatures" and end-product masses. Hence the effects of changing reaction conditions such as reactant concentrations, temperature, and initiators can be quickly assessed. Such a technique is expected to be of wide utility in characterizing polymerization reactions, both on the laboratory scale, where new polymers are synthesized and conditions optimized, as well as on the industrial scale, where on-line quality control can be performed. For stepwise reactions, the RI and TDSLS detectors are sufficient for determination of M-W, whereas for free radical reactions, the polymer concentration must be measured in order to obtain the traditionally defined M-W (i.e. without monomer taken into consideration). The latter was achieved for poly(vinyl pyrrolidinone) polymerization by measuring the monomer concentration with the UV detector. As a further measure of characterization, a single capillary viscometer was also placed in series with the other instruments. This allowed the reduced viscosity to be monitored simultaneously.