in order to determine the electrical isolation mechanism of low energy ions impinging on GaAs heterostructures, a systematic magnetotransport study was performed on the two-dimensional electron gas (2DEG) of a GaAs modulation doped heterostructure. Sample devices were bombarded with 150 eV Ar+ ions while the room temperature resistances were measured in situ. The effectiveness of photoresist as a masking material was tested. Magnetotransport measurements were carried out in a dark condition at 4.2 K, and the resistance, mobility, and carrier density of the 2DEG were monitored as functions of the ion exposure time. The isolation results are consistent with a screening model for the 2DEG electrons. In this model, the onset of electrical isolation is due to the breakdown of the screening because milling of the surface results in depletion of the 2DEG carriers. At present, no additional effects of the low energy ions, such as material damage from ion penetration, have been confirmed