Abstract. Let A 0 be a possibly unbounded positive operator on the Hilbert space H, which is boundedly invertible. Let C 0 be a bounded operator from D(A 1/2 0 ) (with the norm z 2 1/2 = A 0 z, z ) to another Hilbert space U . In Part I of this work we have proved that the system of equationsdetermines a well-posed linear system Σ with input u and output y, input and output space U , andMoreover, Σ is conservative, which means that a certain energy balance equation is satisfied both by the trajectories of Σ and by those of its dual system. In this paper we show that Σ is exactly controllable if and only if it is exactly observable, if and only if it is exponentially stable. Moreover, if we denote by A the generator of the contraction semigroup associated with Σ (which acts on X), then Σ is exponentially stable if and only if one of the entries in the second column of (iωI − A) −1 is uniformly bounded as a function of ω ∈ R. We also show that, under a mild assumption, Σ is approximately controllable if and only if it is approximately observable, if and only if it is strongly stable, if and only if the dual system is strongly stable. We prove many related results and we give examples based on wave and beam equations. 1. Introduction and main results. This paper is a continuation of our paper [35] in which we have investigated a class of conservative linear systems with a special structure, which occur often in applications. These systems are described by a second order differential equation (in a Hilbert space) and an output equation, and they may have unbounded control and observation operators. The main aim of [35] was to prove the wellposedness, conservativity, and other regularity properties of such systems. Here we investigate conditions under which such systems are exponentially stable or strongly stable. It turns out that these stability properties are equivalent to certain controllability and observability properties as well as to certain estimates.We recall the construction from the paper [35] in order to be able to state the new results. Let H be a Hilbert space, and let A 0 : D(A 0 ) → H be a self-adjoint, positive, and boundedly invertible operator. We introduce the scale of Hilbert spaces H α , α ∈ R, as follows : for every α ≥ 0, H α = D(A