Longitudinal velocity and attenuation of the unreacted diglycidyl ether of bisphenol-A and dimethylbenzylamine mixture have been measured for 1 MHz ultrasonic frequency. These are combined with our earlier data [Alig et al., Acta Polymerica, Vol. 40, p. 590 (1989)], which were measured during the curing process of the thermoset, and then analyzed in terms of a process involving a negative feedback between molecular diffusion and chemical reaction. The complex plane plots of the real and imaginary components of the longitudinal modulus are fitted to a stretched exponential relation, and the structural relaxation time is calculated at different stages of curing at temperatures in the range 333-403 K. The results are compared against the calorimetric, dielectric, and viscosity measurements and it is shown that, at least for two cases, gelation time can be observed from the longitudinal modulus data.