 The separation of acetylene and carbon dioxide by porous materials requires delicate control over the pore size. Herein, the authors fine-tune the pore size at sub-nanometer scale in a series of iso-articular metal-organic frameworks to control the acetylene-carbon dioxide separation performance. Subtlestructural differences lead to remarkable performance enhancement. This article was authored by Jiang Wang, Yang Zhong, Yun Su, and others.