Synroc formulations containing 10 wt% simulated PW-4b type HLW in combination with a single transuranic dopant (Np, Pu, Am or Cm) were fabricated and characterised using alpha-track methods, SEM, AEM and dissolution testing. The Pu, Am and Cm-doped samples were calcined in 3.5%H2/96.5%N2. One of the Np-doped Synrocs was calcined in an atmosphere of pure H2, whilst another Np-doped Synroc was calcined in a mixture of 3.5%H2/96.5%N2. All the different formulations displayed microstructures similar to those of Synroc containing actinide-free simulated HLW. AEM derived phase compositions and phase distributions are given. All the Synrocs showed good durability when exposed to leachants (including doubly deionised water, and pH- and Eh-buffered solutions with pHs between 2 to 10). There were no obvious differences between the two Np-doped samples. The improved appearance and behaviour of allthe Synrocs in this study compared with similar formulations described by other authors is attributed to improved mixing techniques during fabrication.