CdTe/Mg x Cd 1-x Te double heterostructures (DHs) grown on InSb (001) substrates using molecular beam epitaxy have demonstrated very long carrier lifetime and low interface recombination velocity (IRV) due to the effective carrier confinement and surface passivation provided by Mg x Cd 1-x Te. However, both thermionic emission and tunneling effects can cause carrier loss over or through the Mg x Cd 1-x Te barriers when the barrier potential is low or when the barrier is thin. Thus carrier lifetime measurement can only give an effective IRV, which consists of the actual IRV that is purely due to recombination through interface trap states, and carrier loss due to thermionic emission and tunneling. By conducting temperature dependent carrier lifetime measurements, the thermionic emission induced interface recombination can be distinguished. Also by comparing samples with different barrier layer thicknesses, the contribution to effective IRV from tunneling effect can be quantified. When both thermionic emission and tunneling effects are eliminated, the actual IRV is measured to be ∼1 cm/s and a very long carrier lifetime of 3.6 μs is observed.