To reduce the pollution of air by minimizing evaporative emissions from fuel tanks, new plastic fuel tanks made of materials with excellent barrier properties have to be developed. Single-layer polyamide 6 tanks are one option to meet the requested low-hydrocarbon permeation rates for motorcycle vehicle tanks. Recently, some problems with respect to deposits in polyamide 6 tanks, blocked nozzles, tubing, and gasoline filters were observed. Thus, samples (precipitates) were taken from unused tanks after conditioning as well as of used tanks and filters after being in contact with gasoline for some time. By investigating the precipitates and deposits by means of infrared (IR) spectroscopy, the main constituents were identified to be cyclic caprolactam oligomers. Additional investigations on the extracted samples by mass spectroscopy allowed us to attribute specific features of the IR spectra to the individual cyclic oligomers (dimer, trimer, and tetramer). In addition, we could show that the crystalline precipitates and deposits in the fuel systems of used vehicles consist of mixtures of the cyclic dimer, trimer, tetramer, and even pentamer of caprolactam with varying contributions of the individual oligomers in dependence of the history of the part. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2012