Double-bond isomerization is a dominant side reaction in hydroformylation of long-chain linear olefins. This isomerization reaction was studied experimentally and theoretically using n-decenes as linear long-chain olefin representatives. On the experimental side, an isothermal batch isomerization experiment was carried out using Rh-BIPHEPHOS as catalyst until the reaction equilibrium was reached. Complete resolution and quantification of all n-decene isomers was achieved using gas chromatography. The observed equilibrium composition was used to extract Gibbs free energy changes of internal n-decenes relative to 1-decene. These were compared to (a) results based on Benson's group contribution method (BGCM) and (b) results from quantum mechanical (QM) calculations. Density functional theory (DFT: BP86, B3LYP, M06-2X) and second order Moller-Plesset (MP2) calculations showed a mean deviation to experimentally determined Gibbs free energy changes of 3-4kJ mol(-1). Using dispersion correction (D3) in DFT calculations reduced the deviations by 1 kJ mol(-1). With a mean deviation of only 0.5 kJ mol(-1) for Gibbs free energy changes, BGCM was closer to experimental data. Entropy errors in QM calculations were estimated to be responsible for approx. 80% of Gibbs free energy errors. (C) 2016 Elsevier B.V. All rights reserved.