The dissociation of ammonium (NH₄⁺) into ammonia (NH₃) in wastewaters is a key factor governing atmospheric nitrogen volatilization. Relatively rich in total ammoniacal nitrogen (TAN or NH₄⁺ plus NH₃), livestock manures are most susceptible to NH₃ volatilization, although indirect measurements report 5 times less NH₄⁺ dissociation as compared to theoretical values. The objective of this study was therefore to directly measure NH₄⁺ dissociation of two standard NH₄Cl solutions (1750 and 3500 mg TAN/L), and of swine and dairy cattle manures at various dilutions and temperatures using a ammonia selective electrode (hydrogen and silver-silver chloride electrode couple) at various pH and temperatures. All solutions demonstrated NH₄⁺ dissociation varying from theory, especially because of dissolved compounds such as atmospheric CO₂. At a neutral pH, ratios of theoretical to measured [NH₃-N] ranged from 1.5 to 3.5, with higher ratios corresponding to higher TAN levels. At a pH below 6, NH₃ volatilization was enhanced by the shift of HCO₃^- to H₂CO₃ and CO₂. With previous research projects reporting 5 times less NH₃ volatilization as compared to theory, the present indicates that dissociation activity account for half of this drop with gas diffusion accounting for the other half.