In the past, electrowetting was usually analyzed on large solid surfaces. In the current study, the effective solid-liquid interfacial tension in a nanoporous silica, which is measured by the ion transport pressure, is investigated experimentally. The interfacial tension decreases as the applied potential difference increases, while the magnitude of variation is much smaller than its bulk counterpart. The effect of the external electric field is saturated at a relatively low voltage. These unique phenomena can be attributed to the confinement effect of nanopore walls. © 2009 American Chemical Society.