An idea about influence of excited molecules on positronium (Ps) formation is proposed, it says: (1) in addition to the excess electrons, some excited molecules created in the spur can also promote the formation of Ps; (2) the excited molecule reaction may exert an influence on the probability of Ps formation by some way depending on the reaction mode (quenching of excited energy or transport of electron from excited molecule to additive) . According to the competitive reaction mechanism in the spur, a relationship correlating the probability of O-Ps formation(I,3,) with the additive concentration (which is included in F and F,*,) is obtained for binary systems (See E,q,(11)) . By this equation, the theoretical fittings are made for some mixtures consisting of a nonpolar substance and an additive such as CS,2, SF,6, C,2,H,5,Br, CCl,4,. From the fitting parameters, some valuable information about the electron affinity of the additive and the ionization potential of the excited molecule are obtained. Furthermore some "knotty problems" in positronium chemistry are satisfactorily interpreted.