The polymerization of acetylene, using [Rh(1,5-Cod)Cl](2), where 1,5-Cod is cis,cis-cycloocta-1,5-diene or [Rh(NBD)Cl](2), where NBD is bicyclo[2,2,1]hepta-2,5-diene, has been followed by u.v.-vis. spectroscopy in solution. The growing polyacetylene was identified by three fine structure maxima in the spectrum, at 500, 544 and 590 nm, which were obtained by subtracting the catalyst spectrum from that of the reaction mixture. In addition, by using the first-order derivative of the absorption spectrum of the growing polyacetylene it is possible to detect vibrational maxima at 480, 515, 550 and 600 nm for the cis isomer and at 640, 670 and 710 for the trans isomer. U.v.-vis., as well as Fourier transform infra-red (FTi.r.) spectroscopy, was used to study the structure of the thin free-standing films of cis-rich and trans-isomerized polyacetylene obtained by using Rh(I) complexes. The absorption spectrum of thin films of polyacetylene produced in this way reveals the absence of the vibrational structure which has been detected in ethanolic solutions during acetylene polymerization. The microstructure of the polyacetylene is very similar to that formed when using a Luttinger catalyst, in terms of defects, which are present as sp(3) hybridized carbon atoms in the polymer, and which can be detected by FTi.r. spectroscopy.