Cyber-physical systems consist of interconnected physical processes and computational re- sources. Because the physical world is connected to the cyber world, cyber-attacks can result in damage to the physical system. If an attacker could access control inputs and mask measure- ments, a cyber-attack could damage the system while remaining undetected by plant operators or control systems. By masking certain sets of measurements, an attacker may cause a portion of the state space to become unobservable, meaning that it is impossible to reconstruct those states. This is called an observability attack. A sequential game-theoretic approach is presented to analyze observability attacks. The sequential game consists of alternating defense and attack stages. In each defense stage, the de- fender's strategy set consists of reinforcing all possible combinations of system measurements. In each attack stage, the attacker's strategy set has two components: a reconnaissance component and a measurement-masking component. The attacker's and defender's payo s are quanti ed at the end of each defense-attack sequence using the responses of the observable and unobservable states. The observability attack game is analyzed for two defense-attack rounds for a nuclear balance of plant system. A mixed-strategy Nash equilibrium is identi ed.