Oxidation of nicotine may result in three nitrosamines being formed - 4-(N-methyl-N-nitrosamino)-4-(3-pyridyl)-1-butanal (NNA), nitrosonornicotine (NNN), and 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK). NNK generally is considered to be formed from the nitrosation of pseudo-oxynicotine (PON), an oxidation product of nicotine. PON was found in green tobacco but accumulation of NNK was not detected. The objective of this research was to determine the nitrosation rate of PON to NNK in different conditions. Kinetics of PON nitrosation to NNK was determined at pH levels of 3.0, 5.5, 7.5 at 20°C and 37°C. Measurement of PON and NNK were done by UPLC/MS/MS protocols. Reactions were done in citric acid (0.16M) and disodium phosphate (0.08M) buffers at the three different pH levels. Very little NNK was formed in the two higher pH levels in 24 h. At pH 3 and 20°C with an initial PON of 6.3 nmole ml-1 and 180 µmole ml-1 NO2 a very rapid formation of NNK was measured. Increased NO2 from 25 µmole ml-1 to 250 µmole ml-1 increased NNK about 5X with initial PON of 6.3 nmole ml-1 in 30 min at 20°C. When PON was increased from 0.63 to 25.2 nmole ml-1 NNK accumulation was linear over 30 min at 20°C with maximum NNK accumulation of 3.56 nmole ml-1. The reaction was more rapid at 37°C with about 2X the NNK formed within 60 min. These results indicate that the generation of NNK is pH dependent and temperature can affect the reaction rate from PON to NNK. These conditions will greatly influence NNK formation and accumulation in the green leaf and during curing.