In this work, a procedure to estimate variables of electrochemical systems involving adsorption and absorption of intermediate species coupled to diffusion is presented. The method is model-based and needs potential and current measurements. Since the system is linear and time variant, the well known Kalman’s filter theory is used to estimate the variables. An analysis of theoretical observability shows the procedure success even in the case of constant potential/current. The error estimation converge asymptotically to zero for slow time variations. The procedure is useful to estimate H concentrations in metallic substrates during H evolution and insertion. To illustrate the proposed procedure, state observations of the hydrogen concentration on steel and metal hydride electrode are presented. In the last case, the procedure is used to estimate the state of charge of the electrode.