In this paper, a method is developed to evaluate the reliability of active RFID tags immersed in water. This situation exists in particular when infants wearing anti-rapt tag take bath. This study discusses the effects on the electromagnetic radiation of an emitting tag plunged into a water volume and then proposes a statistical method to predict the radio coverage of the immersed tag. Finally it demonstrates that the RFID technology is barely affected by this issue even though other RFID technologies do not support immersion. Keywords-RFID localization; radio propagation; indoor environment. INTRODUCTIONTo prevent infant abduction, technology based on RFID is a growing application. To control where and when infant is in the maternity ward, RFID tags are attached to infant's ankle and are identified and located by stations installed in the building. Several technologies or strategies have been proposed to detect and locate the RFID tags. The first strategy consists in measuring the RSSI of the ID signal sent by the tag regularly (e.g. every quarter of minute) and deducing infant's positions by RSSI triangulation [1]. Another strategy uses the Time Of Flight (TOF) triangulation usually based on UWB signal [2]. A third strategy is called polling LF-RF: the tag sends its ID through a radiofrequency (RF) link only if it is in the vicinity of a station placed closed to an access door which covers a limited range by a low frequency (LF) signal. Technologies with passive tags and security gate are affected by weaker ranges than technologies based on active tag.