Inks periodically used to mark the inlet faces of prototype catalyst substrates before testing were analyzed by a variety of methods to identify and quantify inorganic atomic constituents. The dominant inorganic constituents in these inks were Al and Ti, which would likely be in the form of Al2O3 and TiO2 after ink application and firing in air, and thus relatively inert as these are often already constituents in many catalyst washcoats. However, potential catalyst poisons, such as S, Cl, Cr, Zn, and Na were also detected in some inks. A simple model was developed to quantify the possible impact of the potentially irreversible catalyst poisons (such as Si, Cl and Na) based on ink loading during application, constituent concentration in the ink, and catalyst active site density. Model results indicate that ink poison concentrations are sufficient to induce a local poisoning event; however, the total estimated impact on the prototype catalyst is only a 1% or less decrease in the number of active sites. Nevertheless, as contaminant concentration in marker inks is typically neither subject to quality control nor routinely monitored, it is likely that these constituent concentrations could vary markedly from batch to batch, and therefore these constituents could have variable impact on a prototype catalyst to which ink is applied.