The ion-selective field effect transistor (ISFET)-based coulometric sensor¿actuator systems have found applications in acid¿base titration and in the construction of a low-drift carbon dioxide and a pH-static enzyme sensor. In this paper a brief review is given of the previously developed ISFET-based sensor¿actuator systems and the newly proposed system which employs a porous nobel metal as the actuator deposited at a short distance over the gate. The advantages and disadvantages of the different systems are compared. Furthermore, an analytical model is proposed for the new system with a porous nobel metal as the actuator. Two ways to make the porous gold actuator over the gate of the ISFET are described. The acid¿base titration is chosen for the model study. The experimental results are in accordance with the theoretical description. Both the theoretical and experimental results show that devices with a different actuator thickness will behave differently. For a thin porous actuator, an approximately quadratic relationship between the titration time and the concentration of the titrated species is predicted by the model and confirmed by the experiment. For a thick porous actuator, titration of a weak acid shows an approximately linear relationship between the concentration of the titrated species and the titration time, which is predicted by the model. The theoretical calculation of the titration curve of a fully dissociated acid requires the use of a general formula derived from the model. The typical titration times are from 0.5 to 10 s, the corresponding concentrations of the titrated species range from 0.5 mM up to ca. 10 mM.