By adding solutions containing catalyst salts to hot monolith catalysts during partial oxidation processes, the salt decomposes instantly and is deposited selectively near the front face. We have used this technique to deposit extremely small amounts of Pt on alpha -Al2O3 monoliths during ethane oxidation to olefins. We find that on this catalyst with H-2 addition, the selectivity to ethylene rises from similar to 65 to over 80% at an ethane conversion of similar to 60% and at complete O-2 conversion. We also examine the addition of promoters including Sn, which gives improved performance compared to the Pt catalyst alone. This appears to be a general effect that could be useful in preparing catalysts with different loadings and distributions in high-temperature processes. It can also be used for rapid and accurate diagnosis of catalysts and additives and to modify catalysts on-line in situations where deactivation or catalyst loss occurs.