This article demonstrates the feasibility of including simple reagent addition models in an existing observable dynamic model of a froth flotation circuit. The existing model has full state observability and parameter identifiability using measurements that are commonly available on flotation circuits. This article qualitatively evaluates the possible impact of varying frother dosage on the model parameters. A Sobol sensitivity analysis indicates that the air recovery model parameters are most influential in the determination of grade and recovery. The model is expanded with two different reagent effect models. Both expansions include mass balance models of the frother concentration in each cell. The first model expands an empirical parameter in the air recovery model, related to the froth height at which peak air recovery (PAR) is achieved, as a linear function of frother concentration. The second model adds a linear frother concentration term to the existing air recovery model to modify the steady-state air recovery directly. Observability analyses of the expanded models show that all states and the important time-varying model parameters are observable (and identifiable) from the available on-line measurements. Most importantly, the frother concentrations are shown to be observable without concentration measurements. Simulations of the model expansions show that the second model can qualitatively predict the impact of increased frother dosage on air recovery, grade, and recovery, while the first model can only predict the correct effect under certain conditions.