Single breath washout (SBW) is an emerging pulmonary function test due to its relative simplicity and speed. Most significant outcome parameter, phase III ‘alveolar’ slope (tracer gas decay during mid-exhalation) is sensitive to ventilation heterogeneity. In this work, a method is presented for SBW-imaging with two tracer gases with inherently different physical properties: hyperpolarized 129Xe and 3He. Findings in our study are in agreement with trends seen in SBW from pulmonary function lab: the heavier, less diffusive gas (129Xe) has a steeper phase III slope. This indicates increased regional ventilation heterogeneity due to a lesser degree of diffusional mixing.