Speciation behavior of trihalomethanes formation in chlorinated waters due to the effects of bromide is mathematically modeled by a three-tier scheme. Equations for the mole fractions of the mono-, di-, and trihalo-intermediates at each tier level in terms of the initial bromide to chlorine molar ratio are derived and verified by comparison with results of other theoretical studies based on an assumption of constant bromination to chlorination reaction reactivity ratio. It is identified that the usage of applied chlorine concentration or available chlorine concentration for calculation of the relative initial bromide to chlorine concentration would yield different reactivity ratio and should be used consistently. The model equations are also tested by laboratory experiments done with source water at a local plant. It is found that equations with an overall bromination to chlorination rate constant ratio of 18.66 can capture the THM speciation behavior of all four tested incubation periods well.