We present the first spectra of Neptune taken with the Spitzer Space Telescope, highlighting the high-sensitivity, moderate-resolution 10–20 μm (500–1000 cm −1 ) spectra. We report the discovery of methylacetylene (CH 3 C 2 H) and diacetylene (C 4 H 2 ) with derived 0.1-mbar volume mixing ratios of (1.2±0.1)×10−10 and (3±1)×10−12 respectively.