Fifty-five fixed sources from industry were tested for CO and NOx emissions, before and after being converted to natural gas. Measured emissions in terms of ppm corrected to 20% excess air and the standard deviation of the sample are summarized. From this darn, emission factors are calculated and compared to the ones reported by EPA AP-42 compilation. In general there is good agreement between both sets of data. Additionally, emission factors in terms of kg of pollutant per MJ of net heat released is calculated in order to compare emissions with different fuels. In general, NOx emissions are reduced with conversion to natural gas, except in high temperature processes. Changes in CO emissions are not clear dire to high dispersion on the data. However it seems there is a potential to improve CO emissions with simple measures.