We have measured the far-infrared cyclotron absorption of a two-dimensional electron systems (2DES), formed in a GaAs/Al0.33Ga0.67As heterojunction, using transmission-mode terahertz (THz) time-domain spectroscopy. The frequency of cyclotron absorption was proportional to applied magnetic field (B-field) across the measurement range, and from this data, the electron effective mass was extracted. The temperature-dependence of the cyclotron absorption was also studied between 1.5-150 K, allowing the dynamics of electron scattering in the 2D electron system to be determined.