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Emission of Ammonia (NH3) from lagoons and other similar structures holding liquid dairy manures contributes to environmental pollution and also lowers the fertilizer-value of the liquid-effluent. In general, NH3 volatilization from these facilities depends on the concentration of free NH3 in the liquid, which is a function of the dissociation of ammonium ion (NH4+). The dissociation of NH4+, on the other hand, is dependent on the: liquid temperature, liquid pH, and concentration of total solids (TS). In this study the ammonium ion (NH4+) dissociation constant (Kd) was empirically modeled at a pH of 9; at four temperatures (5, 15, 25, and 35C), generally experienced in the US Pacific Northwest, and at five TS concentrations (0.5, 1.0, 1.5, 2.0, and 2.5%; w/w) common in flushed-dairy manure. The Kd of NH3 increased 1.61 times for every 10 oC rise in temperature. The data also indicated a linear decrease in the Kd of NH4+ with increase in the concentration of TS in the liquid. The resulting empirical model of the Kd of NH4+ as a function of temperature and TS had a coefficient of determination, R2, of 0.97; demonstrating a good fit to the experimental data. The Kds of NH4+ in the dairy manure liquid were 117%, 87%, 61%t, and 54% compared to the theoretical Kds of NH4+ in pure water at 5, 15, 25, and 35C, respectively. The results of this study emphasize the need for: (i) including both the liquid-TS and the liquid-temperature in models of Kd of NH4+ in livestock wastewaters, and (ii) covering the entire ranges, of both parameters, encountered in the region where the model will be used.