Following 255-nm excitation of CH2Br2 in acetonitrile, the formation and decay of the iso-H2C-Br-Br species is monitored through its visible transient absorption in the 338-612 nm range. The sub-100 ps decay time constant contrasts the previously reported stability of this isomer on a nanosecond timescale in cyclohexane. Different possible decays channels of iso-H2C-Br-Br are simulated using CCSD and CCSD(T) calculations and we found that the isomerization back to the CH2Br2 structure is the lowest energy decay pathway in acetonitrile. The energy barrier for the reverse isomerization process decreases from 10.6 to 3.7 kcal mol(-1), in cyclohexane and acetonitrile, respectively. (C) 2010 Elsevier B.V. All rights reserved.