DNA stretching under fast force clamp (2 ms force step) with dual laser optical tweezers (DLOT, Smith et al., Science, 271:795, 1996) allows to simultaneously access both kinetic and equilibrium aspects of the overstretching transition from the B to S form (Bensimon et al., Phys. Rev. Lett. 74, 4754, 1995; Cluzel et al., Science 271, 792, 1996; Bongini et al., Biophys. J. Suppl. 1, 96, L.A. 2009). l-phage DNA, (bathing solution 150 mM NaCI, 10 mM tris-HCl, 1 mM EDTA, pH 8.0) was stretched and released with a staircase of force steps of 1-2 pN amplitude at 5, 15 and 25C. Direct inspection of the elongation size and rate at fixed loads at the three temperatures provided all the necessary parameters for a phenomenological two-state model able to reproduce the out-of-equilibrium properties of the system. The results indicate that the number of base pairs undergoing a cooperative stretching transition (the cooperativity index) is 19 ± 3 and that, while the free energy difference between the unstretched and the stretched form is mainly enthalpic in character (DH ~15 kJ/mol with DF~10 kJ/mol), the free energy barriers for the transitions are largely determined by an entropic contribution of TDS ~18 kJ/mol in the stretching direction (DF~23 kJ/mol). Supported by MIUR and Ente Cassa di Risparmio di Firenze (2007.1421), Italy