This paper details the investigation of the properties of inlet gases and shows how they affect the flow patterns immediately in front of the catalyst and the subsequent loss of efficiency. A thorough analysis of the flow distribution at the inlet of the catalyst enabled the effective catalyst diameter to be calculated. Subsequent calculations were then carried out to determine the loss of catalyst function through flow maldistribution.Experimental work involved flowing engine proportioned amounts of air through canisters of a fixed geometric profile containing a catalyst. Inlet cones of angles 10°, 15° and 45° were flowed to estimate the effect of the cone design on the velocity distributions at the face of the catalyst. Simple geometric profiles were investigated to allow a thorough understanding of the mechanism of flow to be comprehended and its affect on catalyst conversion to be analysed. A new ratio has been defined to assess the ability of the cones to develop the flow sufficiently enough to considerably increase the catalyst function.Flow distribution has various overtones in the actual durability of the catalyst substrate namely: 1.Partial catalyst usage of theoretical flow area. 2.During intensive periods of hydrocarbon concentration in exhaust gas, after blowdown, inexact canister design will contribute to: a.Large exotherms due to increased localised concentration of fuel enriched gas, leading to excessive degradation and substrate sintering. b.With the concentrated flow profile the catalyst will experience excessive space velocities that can exceed their designed values by as much as 23%. This leads to the presence of unconverted hydrocarbons after the catalyst known as break through. This information when interfaced with a mathematical model of the catalyst causes a closer correlation between previously published measured and predicted results.