Privacy is a major concern in RFID systems, especially with widespread deployment of wireless-enabled interconnected personal devices e.g. PDAs and mobile phones, credit cards, e-passports, even clothing and tires. An RFID authentication protocol should not only allow a legitimate reader to authenticate a tag but it should also protect the privacy of the tag against unauthorized tracing: an adversary should not be able to get any useful information about the tag for tracking or discovering the tag’s identity. In this paper, we analyze the privacy of some recently proposed RFID authentication protocols (2006 and 2007) and show attacks on them that compromise their privacy. Our attacks consider the simplest adversaries that do not corrupt nor open the tags. We describe our attacks against a general untraceability model; from experience we view this endeavour as a good practice to keep in mind when designing and analyzing security protocols.