Cyanopropyne, CH₃–CC–CN, is a simple molecule whose photochemistry is still unexplored. Here we investigate the UV photolysis of this astrophysically significant nitrile trapped in solid argon. The FTIR study was assisted with ¹⁵N-isotopic substitution data and with DFT-level computations including the analyses of ground- and excited-state potential energy surfaces. Cyanopropyne was found to decay mainly via a two-step isomerization process. Infrared absorption spectra evolved to show signals from allenyl cyanide, CH₂CCH–CN, which then further convert into propargyl cyanide, H–CC–CH₂–CN. Some evidence for the presence of allenyl isocyanide, propargyl isocyanide, 3-cyanocyclopropene, and 1,2,3-butatrien-1-imine under particular experimental conditions was also observed. Although cyano/isocyano interconversion has been observed during photolysis of other closely related species in solid argon matrices, including H–CC–CN, no evidence could be found for production of 1-isocyano-1-propyne, CH₃–CC–NC for these experiments.