Vapor pressures of the system nitrobenzene + tetrabutylammonium picrate, which is continuously miscible from dilute solution to molten salt, are presented at 373 K over the full concentration range. The solvent activities in these systems are analyzed with the Pitzer-Simonson model and a local composition model proposed by Chen. Ion pairing has been considered for both models. The Pitzer-Simonson equations based on a combination of Debye-Huckel and van Laar termds accounting for the long-and short-range interactions, respectively, can accurately reproduce the experimental activity data using ion pairing. The activity data can also be fitted by the local composition model using two parameters.