Various 3,3′-methylenebis azulenes were prepared in high yields upon treatment of 1-substituted azulenes with a solution of 0.1–0.2 wt% paraformaldehyde in glacial acetic acid at 25°C for 1 h. Thus, 1-methyl-, 1-isopropyl-, 1,5-diisopropylazulene, guaiazulene, and methyl 2-hydroxy-1-azulenecarboxylate readily provided the corresponding 3,3′-methylenebisazulenes, respectively. Acetic acid which had been refluxed with 5 wt% KMnO4 for 2 h and then distilled according to a purification method was found to contain 0.1–0.2 wt% formaldehyde and exert the same 3,3′-methylenebisazulene formation. 3,3′-Dideuteriomethyl-enebis(guaiazulene) was obtained by employing the similar distillate from acetic acid-d4.