In flow-based mix networks, so-called flow correlation attacks have been proposed earlier and have been shown empirically to seriously degrade mix-based anonymous communication systems. In this paper, we theoretically analyze the effectiveness of a mix network under flow correlation attacks. Our formulae clearly show how a mix network will ultimately fail when an adversary has access to sufficiently long flow samples, independently of the type of flows (TCP or UDP).We illustrate the analysis methodology by modeling a continuous-time mix, which randomly delays each incoming packet. Our queuingmodel- based analysis captures the essence of flow correlation attacks and can provide useful guidelines for designers who develop and deploy anonymity systems.