A series of nonlinear state observers are developed to estimate unmeasurable variables in a Proton Exchange Membrane Fuel Cell (PEMFC) system. A study of the most relevant variables to estimate which will improve the e ciency of the system and its performance is done. The implementation of the observers is based on the partial di erential equations that de ne the dynamics of the plant, represented by a distributed parameters model. Forward and backward discretization of the distributed model are performed to take advantage of the boundary conditions of the problem. A super-twisting corrective control action is implemented in the nonlinear observers to reduce the estimation error to zero in a finite amount of time. Simulation results are presented to show the performance of the observers to recover the values of the gas species concentrations and to extract conclusions for future research work.