We study the temperature dependence of the luminescence decay of single CdSe/ZnS quantum dots between 2 and 140 K. For the first time, we observe a biexponential decay which was completely hidden in ensemble measurements. We find that the long time component strongly depends on temperature. This demonstrates that the band edge luminescence arises from two thermally mixed fine structure states, the dark ground state and the lowest bright one. To interpret our results, we derive the analytical expressions for the decay using a three level model. Fitting the experimental data leads directly to the lifetime of the states as well as their energy splitting.