The integrated infrared intensities of the acetylenic carbon—carbon and carbon—hydrogen stretching bands have been measured for a series of meta and para substituted phenylacetylenes. The C≡C band intensity is very strongly dependent on the substituent, whereas the ≡C–H band is relatively insensitive to substitution. The C≡C band intensity is insensitive to solvent effects; the intensity of the ≡C–H band is increased slightly in basic solvents. A simple π-electron molecular orbital model for the phenylacetylenes yields results for the dependence of the C≡C band intensity on substituent which are in good agreement with experiment. The analysis shows that most of the intensity in the C≡C stretching band results from transfer of π electronic charge between ring and ethynyl groups during vibration.