We present a detailed theoretical study on nonlinear transport and optical properties in two-dimensional semiconductor systems (2DSSșs) subjected to intense terahertz electromagnetic fields. By solving the momentum- and energy-balance equations using a steady-state Boltzmann equation approach, where electron interaction with LO phonons is taken into account under the lowest-order approximation for GaAs-based 2DSS"s, we have investigated the dependence of electron temperature, momentum relaxation time, and conductivity on the intensity and frequency of THz radiation fields. The results obtained from this study are in line with those obtained from recent experimental measurements [N. G. Asmar et al., Appl. Phys. Lett. 68, 829 (1996)]. © 1997 The American Physical Society.