Upon varying the thickness of trans-polyacetylene layers on different substrates we find a remarkable dependence of the electronic and vibronic properties on the optical density of the films. This behaviour is unaffected by the choice of the substrate or the film-producing process. The spectroscopic results in the ultraviolet, visible and in the infrared (IR), as well as photomodulated absorption measurements in the IR, reveal an improved order in the samples with decreasing film thickness. We find a narrowing of the distribution of the effective conjugation lengths (l(c,eff)) and a shift towards longer l(c,eff) for very thin films ( < 60 nm). To find quantitative relations between l(c,eff) and the electronic properties of our samples we compare them with data on polyenes.