Fossils are frequently conserved with cyanoacrylate (CA) adhesives, which have never been scientifically assessed for their long-term stability and suitability for this application. The degradation of three types of CA adhesives were studied: an ethyl CA, an ethyl CA with added poly(methyl methacrylate) (PMMA), and a butyl CA, in the presence and absence of five different fossils obtained from various sites. The fossils were characterized by pH, moisture content, porosity, and ash content, as well as by their mineral and elemental composition. Hydrolytic degradation of polymerized CA adhesives was monitored by quantitative determination of formaldehyde, one of the degradation products. Both in the presence and absence of a fossil, butyl CA degraded more slowly than ethyl CA, making it attractive for fossil applications. It was also found that acidic fossil inhibits the degradation of CA adhesives, while neutral or alkaline fossils increase the CA degradation. The CA degradation appears to be correlated to some degree with a fossil's physical and chemical properties, but this requires further study. Further study is also necessary to determine how the observed CA degradation affects the actual fossil/CA bond strength.