The stoichiometry of single ternary III-V semiconductor nanowires was analyzed by Raman spectroscopy. Free-standing Al{sub x}Ga{sub 1-x}As nanowires were obtained through metal organic vapor phase epitaxy (MOVPE) by the vapor liquid solid (VLS) mechanism on GaAs substrates. The as-grown nanowires were studied with a scanning confocal Raman spectrometer (spatial resolution {approx}300 nm). They were located by Rayleigh imaging and individual nanowires selected for Raman spectroscopy. The acquired spectra exhibit 2-mode behavior. The stoichiometry of single nanowires was determined based on the frequencies of the GaAs- and AlAs-like transversal optical (TO) and longitudinal optical (LO) peaks with an accuracy of below 10%. Measurements at different positions along the axis revealed variations of the composition within single nanowires. This non-uniformity evidences that the nanowires possess an internal structure. (Abstract Copyright [2010], Wiley Periodicals, Inc.)