Understanding the mechanism of the formation of impurities in pharmaceutical intermediates and starting materials is crucial for a successful control strategy in the manufacturing of active pharmaceutical ingredients. This paper describes how amines containing residual dichloromethane can form substantial levels of formaldehyde during short-term storage. An investigation involving 22 different amines presents evidence underpinning the role of dichloromethane (DCM) in forming formaldehyde. Additionally, control experiments combined with existing knowledge on the reactivity of DCM with amine nucleophiles provide a mechanistic discussion on the generation of formaldehyde via known adducts from the reaction between amines, dichloromethane, and water. Finally, a case study involving a key intermediate of a drug candidate under investigation at Bristol Myers Squibb demonstrates the impact of residual DCM-derived formaldehyde in amine starting material on the formation of a daughter impurity during an amidation step.