Environmental concerns and regulations have stimulated the study of applying catalytic emission control to 4-stroke air cooled utility engines of less than 25 Hp. These engines require air/fuel mixtures considerably richer than those of automotive engines, entailing different catalytic solutions. In addition, small utility engines are subjected to a variety of unique operating modes.Factors discussed for this new catalyst system application are space velocity, temperature, test cycle, operating modes, lube oil consumption, engine control systems, engine life, and operating efficiency as well as other factors unique to this engine. An unexpected effect of this catalyst application, after-ignition of unburned exhaust components in a classical diffusion flame, is also discussed.It appears that catalytic emission control of small 4-stroke utility engines can be effective. However, considerably more work needs to be accomplished before a catalytic system, which is integrated into these utility engines, can achieve the final result -- a clean, safe engine for both the consumer and the environment.