The ideality factor plays an important role regarding the performance of the photovoltaic (PV) system. Even though several works in literature consider that it remains constant during the system operation, such parameter tends to assume reduced values over time, thus degrading the power that can be extracted from the PV module. It is directly affected by the shunt resistance and, particularly, by the increase of the series resistance due to aging. Besides, it is also influenced by weather conditions, thus causing the corrosion of the PV module cells, manufacturing errors, and "hot spots". Within this context, this paper analyzes the behavior of the ideality factor as influenced by the series resistance. An analytical expression is also presented, as it is possible to evaluate the behavior of the PV module when the ideality factor is reduced as a direct consequence of the aging process. Simulation and experimental results are presented and thoroughly discussed.