The dissociation constant of ammonium ion both in deionized water and swine anaerobic lagoon liquid was determined experimentally in a convective emission chamber at three temperatures (15 degrees C, 25 degrees C, and 35 degrees C) commonly experienced in lagoons in the south and southeastern regions of the U.S. Ammonium chloride (NH4Cl) salt was used to make the solution for the deionized water tests. The dissociation constant (Kd) values obtained for NH4Cl in deionized water approximately doubled with every 10 degrees C increase in liquid temperature from 15 degrees C to 35 degrees C. A similar trend was obtained for lagoon liquid in the 25 degrees C to 35 degrees C liquid temperature range, but the Kd values for the lagoon liquid were ~50% of those for NH4Cl in deionized water. However, at 15 degrees C, the Kd value for the lagoon liquid was almost the same as for deionized water, and was 0.75 the lagoon liquid value at 25 degrees C. Based on these results, it can be concluded that the Kd values of ammonium ion in anaerobic lagoon liquid was 50% of the value in deionized water at 25 degrees C and 35 degreesC, and 94% of the value at 15 degrees C. This implies that for lagoons with characteristics similar to those of the anaerobic lagoon liquid reported in this study, the Kd values (normally derived from NH4 + dissociation in deionized water) used in ammonia volatilization calculations should be adjusted to a fraction of that in deionized water. More studies to determine the Kd values for lagoon liquids with different total ammonia nitrogen concentrations and solid contents are needed. Studies should include the effects of temperature and perhaps distinguish between the effects of dissolved and suspended solids on the dissociation constant.