Abstract: This study presents a Java-based real-time Internet access estimation tool for Quality of Service (QoS) in Internet accesses for Multimedia applications (JEQoSIM). It is specially aimed for real-time multimedia applications which use the User Datagram Protocol (UDP). The system is capable of estimating access capacity, available bandwidth and delay as the critical end-to-end QoS parameters for this kind of applications. The algorithm used for QoS estimations is one-way and is based on the packet train technique. Real-time QoS estimation elements are distributed among a central server and the Internet end user. The central server contains a UDP packet bursts server and a web server that hosts the Java applet that implements the UDP packet bursts client. JEQoSIM has been validated using several commercial Internet accesses with different technologies: Asymmetric Digital Subscriber Loop (ADSL), General Packet Radio Service (GPRS) and Universal Mobile Telecommunications System (UMTS).