Aldehyde or ketone 0-[(3-indolyl)(aryl or heteroaryl)methyl]oximes which are useful as color-formers in pressure-sensitive carbonless duplicating systems and thermal marking systems are prepared by reacting 3-[(arylsulfonyl)(aryl or heteroaryl)methyl]-1H-indoles with aldehyde or ketone oximes in the presence of a base.