Analysis of geometric parameters of butadiene and acrolein reveals the contradiction between the Csp(2)-Csp(2) bond length in acrolein and classical concept of conjugation degree in the polarized molecules. In this Letter the reasons of this contradiction have been investigated. It is concluded that the Csp(2)-Csp(2) bond length in acrolein is determined by influence of the bonding for it pi-pi conjugation and antibonding n -> sigma* hyperconjugation between the oxygen lone pair and the antibonding orbital of the single bond. It was shown also this bond length depends on the difference in energy of conjugative and hyperconjugative interactions. (C) 2012 Elsevier B. V. All rights reserved.