Abstract Zhang Zai affirmed the intriguing proposition that, “Tai xü ji qi” (Supreme emptiness is just qi). However, there is little agreement as to the meaning and significance of this proposition. This study first identifies three main interpretations of this proposition in terms of a) “materialism”, b) “synthesis of reality and appearance”, and c) “unity of taixü and qi”. Next, finding serious problems with the “materialism” and “synthesis of reality and appearance” accounts, it goes on to demonstrate that Tang Junyi’s reading in terms of “unity of taixü and qi” offers the best explanation of Zang Zai’s proposition. Finally, drawing upon Tang’s “unity of taixü and qi” reading, this study concludes that the terms “taixü” and “qi” are best understood as referring to two aspects of the same thing.