In its reactions with the title compounds, formic acid variously acts as a formylating, methylating, and reducing agent. Both pyridine and piperidine are converted in significant amounts into 1‐methyl‐, 1‐ethyl‐, 1‐propyl‐ and 1‐pentyl‐piperidines. Of the N‐alkyl groups, isotopic labeling shows that only N‐methyl derives from the formic acid, while the N‐ethyl and N‐propyl arise from heterocyclic ring C‐C bond scission by retro‐vinylogous‐bis‐aza‐Aldol reactions. Detailed analysis of the products for pyridine, piperidine, and their 4‐methyl derivatives, reacted separately and mixed, supports mechanisms in which a piperidine adds 1,2 to a pyridinium cation, or to a di‐ or tetra‐hydropyridine, to initiate reaction sequences leading to the product slates found.