The preparation of dansyl amino acid derivatives is rapid and under carefully controlled conditions is essentially quantitative. The yield is independent of the ratio of dansyl chloride to amino acid, at least over a 1000-fold range. The low recovery of dansyl tyrosine from a chromatogram was avoided when samples were analyzed within one day after derivatization while protected from direct light. Dansyl amino acids show marked changes in retention time and selectivity on reversed phases as the pH of the eluent is altered. A methyl ethyl ketone-2-propanol-based gradient was employed which gave maximum separation between dansyl histidine and dansyl cysteic acid and polar by-products which eluted with the void volume of the column. Dansyl amino acids are highly fluorescent compounds and are readily detected with high sensitivity by fluorescence techniques. An excitation wavelength at 330 nm can be employed to produce a fluorescence emission above 500 nm.