The microstructural changes occurring at the low alloy steel fusion line in nickel based transition welds are believed to be critical to the development of creep cavities and cracks. The progressive changes in Type I carbides observed following laboratory ageing at 625 degrees C and from interrupted creep specimens tested at 590-625 degrees C have been evaluated by optical and electron microscopy. In general, these carbides appear to be M23C6 type and grow in an approximately elliptical shape with the long dimension in the plane of the interface. The behaviour noted is in good general agreement with a cubic growth law and predictions made from laboratory results are in agreement with previous observations from plant welds. The accuracy of these predictions is such that measurements of carbides in service can be used to estimate an equivalent operating temperature.