The BF3‐promoted reaction of paraformaldehyde with the cyclic 1,2‐diketones 1, 3, 8 and 10 was studied. Under identical reaction conditions, the substrates yielded structurally different products depending on their ring size. 1,2‐Cyclopentanedione (3a) was converted into the spiro compound 4 (49%) whereas its 3‐methyl analog 3b gave a butterfly‐like compound 5b (56%). A structurally related compound 2 was obtained (84%) by the reaction of 1,2‐cyclohexanedione (1) with paraformaldehyde. 1,2‐Cycloheptanedione (8) yielded the simple tricyclic acetal 9 (72%) whereas 1,2‐cyclooctanedione (10) underwent a twofold hydroxymethylation in the α‐ and α′‐positions yielding the acetal 11 (53%). The structures of all new compounds were proven by NMR experiments and X‐ray crystallography. The mechanism of product formation is discussed based on steric and electronic arguments. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)