In the previous companion paper, a modified state-surface approach was proposed to algebraically represent unsaturated soil behavior under isotropic stress conditions. The present paper proposes a general theoretic formulation for the modified state-surface approach based on the theory proposed by Fredlund and co-workers. The signs and physical meanings of the material properties, the shapes of the loadingcollapse (LC) and suction increase (SI) yield curves, and the mechanisms used in the Barcelona basic model (BBM) to model unsaturated soil behavior are discussed. Some other problems, such as stress-path independency and its implication on material properties, as well as coupling effects between the LC and SI curves, are also discussed. Based on analysis, it was found that with a minor modification, the modified state-surface approach can be used to study both unsaturated expansive and collapsible soil behavior in a unified framework.