This work documents both experimental data, and by thermodynamic modelling, the synergistic effect occurring in promoted carbon dioxide hydrate systems at the simultaneous presence of tetrahydrofuran and cyclopentane. Cyclopentane has previously been considered a reference among gas hydrate promoters due to its significant pressure reducing effect in hydrate forming systems such as those related to carbon dioxide capture. The present work shows that hydrate dissociation pressures may be lowered by up to 22% compared to those of the cyclopentane promoted carbon dioxide hydrate system by addition of tetrahydrofuran to the aqueous phase. It is shown experimentally that addition of approximately 5 mol% tetrahydrofuran to the aqueous phase of the cyclopentane promoted system, reduces hydrate formation pressures by approximately 20% compared to those of the cyclopentane promoted system at similar temperatures. A thermodynamic model, based on the van der Waals-Platteeuw model and the cubic-plus-association equation of state is applied to model the mixed promoter system. The model accurately predicts the data measured in this work. Furthermore, the model explains the synergistic effect by the fact that tetrahydrofuran displaces cyclopentane from the large cavities of the sII hydrate structure. The most pronounced synergistic effect (largest pressure reduction) is predicted at scenarios, where approximately half of the cyclopentane in the hydrate phase has been substituted with tetrahydrofuran. The model predicts the maximum pressure reduction compared to the cyclopentane promoted system to be approximately 22%. This happens at tetrahydrofuran concentrations of approximately 2.8-3.1 mol% in the aqueous phase, depending on the system temperature. (C) 2014 Elsevier B.V. All rights reserved.