Technical Abstract: Process-based models are being used to predict ammonia emissions from manure sources, but their accuracy has not been fully evaluated for cattle manure. Laboratory trials were conducted to measure the dissociation and mass transfer coefficients for ammonia volatilization from media of buffered ammonium-water solution and dairy cattle manure. Effects of ionic strength, ammoniacal N concentration, temperature and pH of the media, and air velocity over the media were evaluated. As represented in existing models, media type, temperature, and pH were verified to influence ammonia dissociation and mass transfer. In ammonium solution trials, pH was a significant factor in predicting dissociation, but this was not found with manure where pH varied very little among trials. As expected, the temperature of the media and surrounding air had a significant influence on dissociation from both the solution and manure media. Model prediction underestimated measured dissociation values by 5% in solution trials and 94% in manure trials. Further study is underway to explain the high ammonia dissociation measured from cattle manure. Temperature also had a significant effect on measured ammonia flux from both ammonium solution and manure. Although air velocity over the media was expected to affect flux, this was found to be significant in only the trials with ammonium solution. An existing model of ammonia flux predicted emission rates from manure surfaces more accurately than that from ammonium solution with average errors of -16 and +81%, respectively. Temperature had a significant effect on the error between predicted and measured rates with the greatest error at temperatures over 25 deg C. The results imply that model refinements can be made to improve accuracy in predicting ammonia emissions from dairy cattle manure.