This paper presents a novel approach for the frequency domain analysis of a vehicle fitted with a general hydraulically interconnected suspension (HIS) system. Ideally, interconnected suspensions have the capability, unique among passive systems, to provide stiffness and damping characteristics dependent on the all-wheel suspension mode in operation. A basic, lumped-mass, four-degree-of-freedom half-car model is used to illustrate the proposed methodology. The mechanical–fluid boundary condition in the double-acting cylinders is modelled as an external force on the mechanical system and a moving boundary on the fluid system. The fluid system itself is modelled using the hydraulic impedance method, in which the relationships between the dynamic fluid states, i.e. pressures and flows, at the extremities of a single fluid circuit are determined by the transfer matrix method. A set of coupled, frequency-dependent equations, which govern the dynamics of the integrated half-car system, are then derived and the application of these equations to both free and forced vibration analysis is explained. The fluid system impedance matrix for the two general wheel-pair interconnection types—anti-synchronous and anti-oppositional—is also given. To further outline the application of the proposed methodology, the paper finishes with an example using a typical anti-roll HIS system. The integrated half-car system's free vibration solutions and frequency response functions are then obtained and discussed in some detail. The presented approach provides a scientific basis for investigating the dynamic characteristics of HIS-equipped vehicles, and the results offer further confirmation that interconnected suspension schemes can provide, at least to some extent, individual control of modal stiffness and damping characteristics.