An in situ measurement technique that isolates the mobility of charge carriers is described and analyzed. The technique allows significant improvement over conductivity measurements to monitor changes in the physical properties and state of a material as it cures. This is essential in systems where N-i, the number of charge carriers, cannot be assumed constant such as during cure of epoxies, urethanes and polyimides. Currently, there is an assumption made in the literature that the number of charge carriers present in a curing material is constant when conductivity is used as an in situ measurement technique to monitor changes in mobility (and thereby viscosity). This assumption is widely used, for example when dielectric conductivity measurements are correlated with changes in properties such as viscosity. Ion mobility, time of flight (ITOF) measurements, which are described here, are an appropriate technique to isolate and measure particle mobility due to changes in the state of the material. Furthermore, the ITOF technique, coupled with the measurement of sigma, the dielectric conductivity, allows one to measure separately changes in the mobility and the number of charge carriers due to curing or changes in temperature. This is possible since conductivity is the product of the number of charge carriers and their mobility. Length of pulse, strength of applied field, sensor geometry, and temperature/viscosity are examined to determine optimum parameters of measurement for a simple, non-curing system: dimethacrylate of tetraethoxylated bisphenol A (D121). This paper seeks to show that with changes in viscosity, the pulse length and magnitude of voltage in relation to the distance between the electrodes should be varied to obtain accurate ITOF information on the changing mobility. (C) 2003 Elsevier Science Ltd. All rights reserved.