Recent amendments to the Clean Air Act have renewed interest in synthesizing isobutylene from coal derived synthesis gas for use in producing methyl-tertiary-butyl ether. Previous work in isosynthesis was performed at extreme conditions or at very low conversions. This paper reports on both the process conditions and the catalyst that give the highest yield of isobutylene. The catalysts were zirconia based and were synthesized by both a precipitation and a hydrothermal method. The effects of sodium, titanium, manganese, cerium, and thorium additives were investigated. Catalysts prepared by the precipitation method were most active while those prepared hydrothermally were more selective to the i-C4′s. Results for the most active catalyst for each preparation method are as follows. A CO conversion of 35% (34% CH4, 20% i-C4′s, 26% C5+, wt basis) was achieved at 673 K, 50 atm, 1/1 CO/H2 ratio, and 150 sec space time over a precipitated 7% (wt) Ce-ZrO2. A 1.6% Na, 10.3% Ti, ZrO2 prepared hydrothermally gave a CO conversion of 16.7% (37% CH4, 30% i-C4′s, 22% C5+) under the same conditions.