The effect of trace impurity elements on the hot-ductility behaviour of both simple and complex copper-nickel alloys has been investigated using a modified Gleeble apparatus. The elements studied were bismuth, lead, selenium, sulphur, and tellurium added singly and in combination at various levels in the range 1–750 ppm (wt). High strain-rate tensile tests (10S−1) were carried out on the alloys over a range of temperatures during the cooling part of a simulated TIG welding cycle. Ductility was found to decrease progressively as the amount of impurity increased, but recommended safe limits could be specifiedfor simple alloys. Bismuth and tellurium were particularly harmful whereas sulphur was relatively innocuous even at levels as high as 750 ppm. When such elements were present in combination, the ductility was significantly decreased and a simple relationship between ductility and the total trace impurity content of the alloy is produced. This gives reasonable agreement with experimentally determined ductilities of alloys containing two major impurities and of a commercial-purity Cu-10Ni alloy. With more complex alloys the relationship is able to rank alloys of various purities in order of increasing ductility. Significant improvements in ductility in commercial-purity alloys were achieved by using purer melting charges and better melting and casting procedures. Moreover, impurity-alloying-element reactions in such alloys resulted in more severe embrittlement than in simple alloys.