This paper investigates the influence of thermal lag on the glass transition temperature of polymers measured under different heating rates during dynamic mechanical analysis (DMA). The E-modulus-temperature curves of a two-component epoxy adhesive and a pultruded glass fiber-reinforced polyester were measured by a DMA machine. Experimental results showed that two kinds of thermal lag result when the heating rate exceeds 0.5 degrees C/min: one between the specimen temperature and the thermocouple of the machine and the other along the length of the specimen. A finite volume model was developed in order to simulate the thermal environment inside the DMA chamber and validate the measured thermal lags. A numerical procedure was adopted to calculate the influence of the thermal gradient along the length of the specimen on the E-modulus-temperature curve. It was found that when the thermal lag effects are excluded, the E-modulus of a specimen decreases with increasing heating rates. This result contradicts reported results obtained using the experimentally derived E-modulus vs. temperature curves, disregarding the influence of the thermal lags, and thus claiming an increase of the E-modulus with increasing heating rates. (C) 2014 Elsevier Ltd. All rights reserved.