Abstract. IPv6 provides an expanded address space to satisfy the future Internet requirements. In this paper we compare and analyze onemonth measurements of the end-to-end IPv6 delay and hopcount between 26 testboxes of the RIPE TTM project with the corresponding parts in IPv4 network. By comparing IPv6 and IPv4 paths, we focus on problems that are only present in the IPv6 paths. In those poorly performing IPv6 paths, we run traceroute with the path maximum transmission unit (MTU) discovery to identify the problems and their causes.