Carbon-carbon spin-spin coupling constants across one bond have been measured in several mono and dialkyl substituted diacetylenes, in dialkyl substituted tetraacetylene and in trimethylsilyl substituted diacetylene. This provides new examples of the coupling constants across C_sp-C_sp single bonds, which, so far, have been determined for a very few compounds only. The semi-empirical INDO FPT calculations have been performed for mono substituted diacetylenes with substituents varying along the first row of the Periodical Table. The calculated results revealed that one-bond ¹³C-¹³C coupling constants across CC triple bonds with the substituent directly bound to the bond concerned as well as those across the central single CC bond are significantly affected by substituent effects.