Abstract Single determinant ab initio molecular orbital theory has been applied to the description of the equilibrium geometries, total energies, atomization energies, force constants, dipole moments and total atomic charges of the first five cyanopolyynes, H(CC) nCN. Hydrogen cyanide and acetylene are included for comparison. In order to study the basis set dependence of the computed properties, three methods of introducing more flexibility into the STO-3G minimal basis set have been considered: increasing the number of primitives, introducing split-valence basis sets and adding polarization functi The mean absolute deviation between the theoretical and observed bond lengths decreases along the series STO-3G, STO-6G, 3-21G, 4-31G and 6-31G. The relatively new 3-21G split-valence basis set leads to the best agreement with the experimental dipole moments. At the STO-NG level the net atomic charg and equilibrium bond lengths are observed to tend monotonically, as a function of chain length, to certain asymptotic values. Due to cancellation of errors in the CC and CC bond lengths, the STO-3G basis set yields estimates of the rotational constant, B e, for the larger cyano which are expected to be too large by about 0.6%.