An investigation has been made of the reaction of isobutene with mercury (3P1) atoms at 30°C. Acetylene and propylene have been shown not to be among the main products of the reaction. The over-all rate of consumption of isobutene and the rate of pressure decrease both decrease with increasing initial pressure in the complete quenching region. These rates approach constant values for initial pressures of 180 mm and higher The results of this investigation show that isobutene behaves similarly to the other olefins in reacting with mercury (3P1) atoms at 30°C, in that the reactions can be explained by the following initial steps for the mechanism: i−C4H8+Hg(3P1)→i−C4H8*+Hg(1S0)i−C4H8*+i−C4H8→2i−C4H8,accompanied by the decomposition of the activated isobutene molecule into reaction-initiating fragments. The discrepancies between the results of this investigation and those of the earlier work of Gunning and Steacie may be due to the effect of impurities in the original isobutene, since it is shown that small amounts of added propylene or acetylene change the rate of reaction markedly.