Catalyst ink is a key material of the catalyst layer in a proton exchange membrane fuel cell, but can be degraded as the reactions of alcohols are promoted by Pt in the ink. The present study quantitatively determined the concentrations of reaction products in an ethanol/water-based catalyst ink, and the effects of these substances on the ink and catalyst layer were investigated. The acetaldehyde concentration was found to be the key factor, as acetaldehyde caused particle agglomeration in the catalyst ink and micro-scale crack formation in the catalyst layer.