Results are presented on the comprehensive analytical identification of the N-cyclohexylamides obtained as unexpected reaction byproducts in the synthesis of N-phenyloleamide (NPOA) and N-phenyllinoleamide (NPLA) by reaction of anilide with either oleic or linoleic acid. The data presented prove that these N-cyclohexylamide impurities (5-25%) are due to the MN'-dicyclohexylcarbodiimide used as dehydrating agent. The synthesis of both phenylamides was required for toxicological testing in relation to the Spanish Toxic Oil Syndrome. Purification of the reaction crudes by silica gel column chromatography followed by subsequent TLC and HPLC assay of the presumably pure amides did not eliminate the cyclohexylamides, which had not been detected by either technique. Unequivocal identifications were achieved by a combined analytical approach involving packed and capillary GC and GC-MS, chemical ionization MS, IR and NMR spectroscopy, and elemental analysis. On the basis of the above results, an alternative procedure is suggested, leading to the desired amides in high purity and good yields.