We introduce a representative database of 60 accurate diene isomerization energies obtained by means of the high-level, ab initio Wn-F12 thermochemical protocols. The isomerization reactions involve a migration of one double bond that breaks the π-conjugated system. The considered dienes involve a range of hydrocarbon functional groups, including linear, branched, and cyclic moieties. This set of benchmark isomerization energies allows an assessment of the performance of more approximate theoretical procedures for the calculation of π-conjugation stabilization energies in dienes. We evaluate the performance of a large number of density functional theory (DFT) and double-hybrid DFT (DHDFT) procedures. We find that, with few exceptions (most notably BMK-D3 and M05-2X), conventional DFT procedures have difficulty describing reactions of the type: conjugated diene → non-conjugated diene, with root mean square deviations (RMSDs) between 4.5 and 11.7 kJ mol-1. However, DHDFT procedures show excellent performance with RMSDs well below the 'chemical accuracy' threshold. © 2014 Elsevier B.V. All rights reserved.