A spectrophotometric method has been developed for the analysis of crotonaldehyde based on its reaction with 4-hexylresorcinol. The product formed has two analytically useful absorption maxima at 385 and 345 micrometers. The crotonaldehyde product obeys Beer's law in the 1 to 20 microgram ml range. Saturated aldehydes and ketones, alcohols, esters, acids, most hydrocarbons and phenols do not interfere significantly. Unsaturated aldehydes, unsaturated ketone and diolefins in excess interfere appreciably, as does nitrogen dioxide. Results of the analyses of a number of liquid mixtures are given. (Author)