This paper presents a simple stability test method based on changes in carbonyl compounds. Whereas the method of identifying carbonyl groups by means of titration is in itself well-known, its use in determining the stability of fast pyrolysis bio-oils has previously not been established. Pyrolysis oil stability is typically measured as viscosity increase under specified conditions. The increase in viscosity occurs due to an increase in the amount and molecular weight of water insolubles present in the oil. The change in carbonyl compounds is due to reactions of aldehydes and ketones during storage, the reaction products of which end up in the water-insoluble fraction. This paper shows that the carbonyl titration method, based on changes in aldehydes and ketones, correlates with the viscosity increase-based stability test and that, due to this clear correlation, both methods can be used and comparison between the methods is possible.