We consider a time-dependent two-level quantum system interacting with a free Boson reservoir. The coupling is energy conserving and depends slowly on time, as does the system Hamiltonian, with a common adiabatic parameter ε. Assuming that the system and reservoir are initially decoupled, with the reservoir in equilibrium at temperature T ≥ 0, we compute the transition probability from one eigenstate of the two-level system to the other eigenstate as a function of time, in the regime of small ε and small coupling constant λ. We analyse the deviation from the adiabatic transition probability obtained in absence of the reservoir.