Dietary manipulation can substantially lower ammonia (NH3) emissions from laying-hen houses or manure storage. Recent lab studies showed a reduction of 40–60% in ammonia emissions for an experimental (EcoCalTM) diet as compared the standard or control diet. However, adoption of a mitigation technology at commercial production level should be preceded by substantial field verification tests to document not only NH3 emission reduction, but also impact of the strategy on production performance of the hens and cash returns. A study to assess the effects of feeding diets containing EcoCal on NH3, hydrogen sulfide (H2S), and carbon dioxide (CO2) emissions, laying-hen production performance, and economic returns was conducted at a commercial laying-hen farm in central Iowa. Two houses (256,000 or 262,000 hens per house) were used for the study. Hens in one house were fed the EcoCal diet while hens in the other house were fed a standard or control diet containing no EcoCal. A state-of-the-art mobile air emissions monitoring unit (MAEMU) and the associated sampling system were used to continuously monitor the gaseous concentrations, ventilation rate and environmental conditions. Comparative data collected from December 2006 to May 2007 are presented in this paper. Data from this period showed that the EcoCal diet led to NH3 emission reduction by up to 23.2% (0.86±0.04 and 1.12±0.03 g/d·hen for EcoCal and Control diet, respectively), at the same time, H2S emission increased by up to 134% (4.38±0.20 and 1.82±0.07 mg/d·hen for EcoCal and Control diet, respectively), although the magnitude of H2S emission is rather small for both dietary regimens. Data on the hen production performance are reported in a companion paper (Roberts et al., 2008).