Transient reaction steps often remain elusive to conventional NMR detection. Using hyperpolarized dDNP-NMR with signal enhancements over 30 000, we clarified elusive pathways in biomass upgrading. Both furfural and a lactone deriving from furfural were hyperpolarized using a PEG-400/trityl matrix at natural isotope abundance. Hyperpolarization allowed the detection of elusive reaction species by tracking protonated sites that undergo significant chemical shift changes. Combined with careful choice of the decoupling scheme, hyperpolarized NMR revealed that furfural undergoes Baeyer-Villiger oxidation, while the lactone undergoes epoxidation.