Ammonia is the predominant nitrogen-containing species formed during the pyrolysis of oil shale. An on-line method of determining ammonia was developed to study kinetics and chemistry of formation and evolution. While fragmentation of water interferes with electron ionization mass spectrometry, chemical ionization mass spectrometry using isobutane reagent gas gave excellent sensitivity and selectivity for ammonia. All transfer and sampling lines from the retort were heated to ~150supO/C. We successfully measured ammonia evolution from various oil shales retorted under different conditions, including steam retorting.