Surface tension measurements on a polymerizing system can yield information concerning concentration changes of surface-active species present in the system. Such measurements have hitherto been performed under conditions more or less remote from those obtaining during the actual polymerization. The use of a twin-capillary dilatometer permits, in principle, the simultaneous determination of both conversion and surface tension as a function of time. The design of such a dilatometer is described, the precautions associated with its use examined, and its application to the monitoring of the surface tension of various polymerizing systems discussed.