Wireless Sensor Networks (WSNs) can be used to monitor hazardous and inaccessible areas. The WSN is composed of several nodes each provided with its separated power supply, e.g. battery. Working in hardly accessible places it is preferable to assure the adoption of the minimum transmission power in order to prolong as much as possible the WSN's lifetime. Though, we have to keep in mind that the reliability of the data transmitted represents a crucial requirement. Therefore, power optimization and reliability have become the most important concerns when dealing with modern systems based on WSN. In this context, we propose to evaluate the effectiveness of a Transmission Power Self-Optimization (TPSO) technique for WSNs in an Electromagnetic Interference (EMI) Environment. The TPSO technique consists of an algorithm able to guarantee an equally high Quality of Service (QoS), concentrating on the WSN's Efficiency (Ef), while optimizing the transmission power necessary for data communication. Thus, the main idea behind our approach is to reach a trade-off between Ef and energy consumption in an environment with inherent noise. © 2010 IEEE.