Voltage sensing domain (VSD) relaxation is a process involving a voltage-independent transition from the activated state into the relaxed state. This process has been shown to shift the voltage dependence for sensing charge movement (sensing current) in the voltage sensitive phosphatase (VSP) known as Ci-VSP and to slow down the deactivation of potassium currents in Shaker and Kv1.2. Given the effect of VSD relaxation on the dynamics of these voltage sensitive proteins, it is thought that the relaxed and the active states are comprised of different set of conformations. Thus, it is possible that these states are sensitive to different physiological parameters. Here, it is shown that VSD activation and relaxation in Ci-VSP display different sensitivity to external pH (pHEXT). Sensing currents of Ci-VSP recorded during activation slowed down as pHEXT was decreased. Noteworthy, the kinetic of deactivating sensing currents was less affected, suggesting that the initial activation of the VSD involved a pHEXT-sensitive, rate-limiting step. In spite of these observations, the voltage dependence of activating sensing currents was almost unaltered by pHEXT (6.5 to 9.0). In contrast, voltage dependence was strongly modulated by pHEXT after VSD relaxation. In fact, the shift in voltage dependence for sensing current was 30 mV larger at pHEXT 6.5 with respect to 9.0. These observations indicated that the equilibrium between the resting and active states was pHEXT insensitive. Conversely, the equilibrium between the relaxed and the relaxed-resting states was unambiguously pHEXT sensitive. Furthermore, given that decreasing pHEXT slowed sensing currents, this observation suggests that the energy barrier between the resting and active states, rather than the difference in energy between these states, was modulated by pHEXT. In contrast, for the relaxed VSD, pHEXT strongly modulated the difference in energy between the relaxed and the relaxed-resting states.