A unified equation of state theory valid from pure fluids to multicomponent associated mixtures of molecules having any number of hydrogen (proton) donor and acceptor groups was formulated. Under the fundamental assumption that the intermolecular forces are divided into physical and chemical forces, generalized Veytsman’s statistics for hydrogen bonding theory was combined with the nonrandom lattice-fluid theory. By the two-fluid approximation, configurational Helmholtz free energy was derived from the lattice of the Guggenheim combinatory. The Veytsman’s statistics extended for dimers and n-mers was employed for the description of some associating systems including organic acid.