A rapid, mild procedure is described for the synthesis of picolinyl esters of branched-chain, cyclic and unsaturated fatty acids, dicarboxylic acids and hydroxy acids by the reaction of the derived acid chloride with 3-pyridylcarbinol. The mass spectra of these derivatives contained abundant diagnostic ions resulting from radical-induced cleavage of the hydrocarbon chain following random hydrogen abstraction hy the nitrogen atom of the pyridine ring. Ions thus represented cleavage, without further rearrangement, at each carbon-carbon bond and enabled methyl groups and double bonds to be located. The spectra of the polyenoic acids were more complex than those of their monoenoic analogues, but major ions whose mass differences could be represented by the series ---14,26,14,26,14---could be used to localize the double bonds. The position of the double bond most remote from the carboxylic ester group could be established by the position of an abundant ion produced by loss of an allylic radical containing the terminal carbon atoms of the chain. Derivatives of the hydroxy acids, although not prepared in quantitative yield, also gave diagnostic spectra. The spectra of the derivatives of the dicarboxylic acids contained abundant molecular ions and again showed diagnostic cleavages of the hydrocarbon chain.