IPv6 networks are rarely fully IPv6 from end to end particularly when interconnected over other providers core data networks, hence the need for IPv6 transition methods or mechanisms. Previous studies have identified some potential impact on performance where transitions occur. This study considered implementations where Quality of Service (QoS) service policies have been applied in the IPv6 network to determine if the transition mechanisms were able to guarantee the same performance and level of service when the IPv6 traffic transitions over a core IPv4 network. Using a test lab, traffic generators and data capture tools the study was able to fully test the transition mechanisms using data rates and link speeds that replicated equipment and traffic levels used in real world implementations. The study showed that the Quality of Service classes applied were maintained across the transition network providing the service guarantees required for a range of traffic classes. Results indicated that the transition implementations on the devices used (Cisco ISR 4351) translate the QoS settings from IPv6 to IPV4 and vice versa were highly effective and with negligible additional impact on performance occurring due to the additional processing required.