Abstract: A new osmotic-swelling system for montmorillonite with formamide (FA) is described. Water and a variety of polar organic liquids were used to swell FA-montmorillonite complexes. The post-swelling behavior of the complex is interpreted in terms of the bi-functionality of FA as an electron donor/ acceptor and the magnitude of the donor number (DN) of the polar liquid relative to that of FA. The FA-montmorillonite complex shows limited swelling of d(001) <2.3 nm for polar liquids with DNs larger than that of FA. In contrast, the FA-montmorillonite complex shows osmotic swelling when the DN of the polar liquid is smaller than that of FA.