Fusion welding resin articles difficult to melt

In a method of fusion welding resin articles which are difficult to melt due to a fusion retarding element or structure in the composition, a fusion accelerator is applied to the surfaces to be bonded, the accelerator being capable of destroying the fusion retarder when heated. After or simultaneously with this step, the resin articles are heated so that the accelerator destroys the retarder and the surface portions melt and bond before the articles are allowed to cool. The accelerator may be coated directly onto the surfaces to be bonded, or may be applied to an element which is set within the joint, for example on the insulating sheath 4 of a heating wire 3 which is to provide the heat for welding.