The in situ measurement of dynamic changes in viscosity induced by illumination has been performed on a range of photosensitive urethane dimethacrylates (UDMA) evaluating the response at 2 three different illuminations intensities (1, 2, and 5 W cm (-2)) and at three different diluent concentrations (15, 30, and 50%), using 1,6-hexanediol dimethacrylate (HDDMA). The initial viscosity value ranged between 1 and 10 Pa s for the mixtures with final viscosities approaching 1 x 10(7) Pa s after illumination. The initial rise in viscosity was analyzed using an exponential model with an exponent ranging from 1.0 to 2.5 with time under exposure. Higher conversion rates were observed with both higher intensity and lower HDDMA content. The analytical approach taken here could aid in developing more sophisticated models that consider simultaneous flow and cure advancement in other thermosets. (c) 2007 Wiley Periodicals, Inc.