Despite the specific importance of four-membered heterocycles with a carbon−heteroatom double bond at the 3-position in organic and medicinal chemistry, little attention has been given up to now to theoretical computational studies on these molecules. However, the overall geometry, and degree of ring puckering especially, could significantly influence the reactivity and biological properties of these four-membered ring compounds. In this paper, focus is made on the influence of different substituents on the equilibrium geometry, ring puckering potential, and thermodynamic quantities. It was found that these properties are mainly affected by the heteroatom (oxygen, nitrogen, sulfur, phosphorus) contained in the ring skeleton. Moreover, the correct description of the puckering potential with the hindered rotor treatment leads to substantial corrections on the thermodynamic properties in the harmonic oscillator approximation.