The structures of an extended series of cumulenes containing a donor and acceptor group situated at either end of the chain have been calculated using the AM1 method and the 3-21G basis set. The dipole moments increase sharply with increasing chain length and show little sign of saturation even at long chain length. The hyperpolarizabilities of these structures calculated using a sum-over-states method also increase with increasing chain length to give very large values at extended chain length. Related calculations on the donor-acceptor polyenes show that the starting structure adopted for the subsequent calculation of both the dipole moment and hyperpolarizability has a large effect on the result, particularly for long chain systems. At short chain lengths of up to eight carbons, the polyenes are predicted to have larger hyperpolarizabilities than-the cumulenes, but the position reverses with further chain extension; both have greater values than those calculated for the corresponding polyynes.