One of the primary drawbacks in the isosynthesis reaction (synthesis gas to iso-C4 hydrocarbons) is the reduced selectivity to the iso-C4 hydrocarbons because of methane production. The effectiveness of sulfur as a catalyst poison (especially for methanation ) is well documented. Sulfur reduces catalytic activity in almost every case, but selectivity to a desired product can sometimes increase because reactions that form undesired products are selectively poisoned. This paper reports on the effect of co-feeding a one percent hydrogen sulfide in hydrogen mixture in a one to one ratio with carbon monoxide over a precipitated 7 wt.-% cerium zirconia in an attempt to reduce methane formation. When hydrogen sulfide is included in the feed a dramatic shift in product selectivity occurs with no loss in catalytic activity. The Cz + C3 fraction is reduced by about half while the C5 fraction is increased by a factor of two and a half. This increase is realized in the formation of 3-methyl-l-butene, the second least thermodynamically favored C5 hydrocarbon, which shows an eighteen fold increase. Perhaps a suppression of chain growth to higher (C1o+) hydrocarbons, a reduction in the amount of C5 cracking to C2's and C3's, or possibly a steric (or surface) effect favoring the formation of 3methyl-1-butene over the more thermodynamically favorable 2-methyl C5 alkenes is responsible for this phenomenon. Temperature cycling deactivates the catalysts and shifts the product distribution toward the alkanes. However, hydrogen sulfide seems to negate the effects of temperature cycling on the product distribution.