The infrared absorptions of 141 analogues of griseofulvin and isogriseofulvin have been examined, and the behaviour of absorption bands associated with carbonyl, alkyl, alkoxy-, alkylthio-, and halogen substituents is described. Absorption bands that can be used to distinguish between 4'-oxo-2'-enol ether and 2'-0xo-4'-enol ether analogues, and between griseofulvin and epigriseofulvin, are reported. Certain analogies with the spectra of l-methoxycholest-l-en-3-one and 3-methoxycholest-2-en-l-one are discussed.