Dipole moments and molar Kerr constants at 25° are recorded for 3-chloropropyne, 3-bromopropyne, phenylacetylene, diphenylacetylene, di-(p-chlorophenyl)acetylene, and dimethyl and diethyl acetylenedicarboxylate, as solutes in carbon tetrachloride. Anisotropic polarisabilities (bL= 3·79, bT=bv= 1·26) for the C≡C bond, deduced from literature data, are satisfactorily applicable to 3-chloropropyne, 3-bromopropyne, and phenylacetylene. Observations on the esters can be interpreted in terms of a mixture of non-planar cis- and trans-isomers.