A theoretical investigation is presented of the electrohydrostatic stability of a given volume of incompressible dielectric fluid when stressed by the application of a potential difference between bounding conducting fluids. It is assumed that the dielectric fluid is located in a channel of breadth 2 a and height 2h, with h/a [Lt ] 1, whose walls are semi-infinite solid dielectric sheets of thickness 2h. The dielectric fluid may have a volume which differs from that of the channel, so that the presence of menisci at the interfaces between conducting and non-conducting fluids is taken into account. By a suitable method for approximating the electric stress at the interfaces, the electrostatic potential difference across the dielectric is determined as a function of the pressure difference across the interfaces for prescribed values of the discrepancy of the volume of the dielectric from the volume of the channel per unit length, and criteria are obtained for determining the critical electric field which precipitates the instability of the system. The variation of the critical electric field with the dimensionless volume excess 2δ is also found and it is shown that, for δ < −0·5, instability is associated with a symmetric mode of disturbance in which the critical field occurs at the maximum in a plot of potential difference vs. pressure difference. For δ > −0·5, instability arises from an asymmetric disturbance with the critical field occurring at a bifurcation point in the potential difference/pressure difference plane. Bifurcations are shown to occur only when the equilibrium profiles of the interfaces have extrema at the edges of the channel.