Significance Nonheme iron enzymes FTO and ALKBH5 play central roles in epigenetic RNA regulation by catalyzing the oxidation of N 6-methyladenosine (m6A) to produce N 6-hydroxymethyladenosine (hm6A) and adenosine (A), respectively. Here, we provide a mechanistic rationale for these distinct biochemical outcomes by identifying that ALKBH5 performs m6A demethylation via an unprecedented covalent-based mechanism with concomitant and rapid release of A and formaldehyde (FA), whereas FTO liberates hm6A to release A and FA over longer timescales. This work reveals foundational biochemical differences between these closely related but nonredundant epigenetic enzymes and identifies ALKBH5 as an endogenous source of rapid formaldehyde generation in cells.