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An instrument for the reproducible measurement of polymerization shrinkage kinetics is described, constructed around a disc-shaped specimen sandwiched between two glass plates. Test specimens of light-activated resins were irradiated through the lower, rigid plate. The physical response of this measurement system, in relation to the characteristics of photo-polymerized materials, has been subject to an original detailed analysis and validation. The upper non-rigid plate was readily deflected by an increase of the adhesive stress from the polymerizing and shrinking sample. Deflection was measured by a LVDT transducer and computer recorded. Dimensional changes were confined to the specimen disc-thickness dimension, such that the fractional linear shrinkage approximated the volumetric shrinkage. Shrinkage data are reported for representative unfilled and composite resins. Equilibrium shrinkage magnitudes ranged from 1.3 to 7.9%. The kinetic behaviour was approximately characterized by an overall time constant, ranging from 12.5-27 s, associated with an exponential growth curve, although the initial shrinkage was nearly linear in time, for many materials, due to non-steady-state concentrations of polymer free radicals.