The advantage of a high signal-to-input power and path-length ratio, while retaining very good flexibility of the operation, has made the recently developed gasphase mirage-effect spectroscopy setup an excellent tool for the measurement of trace gases and organic volatiles up to ppb or lower concentration levels. The present study demonstrates an unusual application of this novel technique in the area of the nondestructive characterization and photodecomposition of polymers. Very small quantities of ethylene gas were found to be released when polyethylenes were exposed to a very low power mercury lamp (photoexposure). The extent (quantity) of the ethylene gas produced was found to be specific and characteristic of the type of polyethylenes (e.g., HDPE and LDPE). The source of ethylene was attributed to the rupture of the terminal monomer units of the branched chains. This study, apart from fundamental understanding, also demonstrated a direct industrial application in the fast and nondestructive characterization of various polyethylenes.