In addition to the previously reported products dehydroacetic acid (1) and a pyranopyran derivative, presently revised as 4,7-dimethyl-2H-pyrano-13,2-c]-2H-pyran-2,5-dione (2), seven new polysubstituted coumarin derivatives have been synthesised by NaHCO3 catalyzed self-condensation of ethyl acetoacetate. The new products have been characterized as 3-acetyl-6-carboethoxy-5-hydroxy-4,7-dimethylcoumarin (3), 5-hydroxy-4,7-dimethylcoumarin (4), 3-acetyl-6-carboethoxy-4,5,7-trimethylcoumarin (5), 6-carboethoxy-4,5,7-trimethylcoumarin (6), 9-carboethoxy-10-hydroxy-2,4,8-trimethylbenzo-[3,4]-coumarin (7) and 8-carboethoxy-3,7,9-trimethylfurano-[3,2-c]-coumarin (8) based on their 1H and 13C nmr, and mass spectral studies in conjunction with the mechanistic rationale of their formation and some chemical evidence. In the same way the remaining new product is tentatively assigned 3″-carboethoxy-7,9,10,10′-tetramethyl-tetrahydrofurano-[2,4-de]- 4′,10′-dihydro-[5,6]-naphthopyran-2,5-dione (9) structure. Transformations of 3 to 4, 5 to 6 (minor), 6-carboethoxy-4,5,7-trimethylcoumarin (10) and 4,5,7-trimethylcoumarin (11), and 6 to 11 have been effected by heating with conc. H2SO4. Use of 60% H2SO4 increased the yield of 11. Decarboxylation of 10 affords 11. Sodium bicarbonate mediated self-condensation of methyl acetoacetate affords the methyl ester 12 of 10. Oxidation of the 4-methylcoumarins 6, 11 and 12 with SeO2 gives the corresponding 4-formyl and 4-hydroxymethyl derivatives. The one-pot monosubstrated oligomerization of ethyl acetoacetate thus demonstrates a simple biomimetic type synthesis of a number of new dimethylcoumarin and trimethylcoumarin derivatives.