Charpy-V specimens of several test welds made of MnMo, MnCrMo, MnNiMo and MnNiCrMo types were irradiated simultaneously in two series at 315°C and 285°C, respectively, with neutron fluences between 8 × 10¹⁸ and 5 × 10¹⁹ n/cm² (E>1 MeV). The known influence of copper on the 41 J temperature shift (ΔT₄₁) was clearly detected for variations from 0.08 to about 0.3%. A considerable variation in embrittlement of low copper (below 0.08%) welds was not traceable to copper or phosphorus. Obviously it is related to variations of the nickel content (up to 1.7%). The effects of copper and nickel seem to be independent on each other. Both, however, have an influence on the slope of the ΔT₄₁ against fluence curves. The effects of other elements as silicon, phosphorus, arsenic, manganese, chromium, molybdenum and of welding procedure and welding flux are discussed in the paper. Due to over-lapping, these effects could not be clearly defined. The Charpy-V upper shelf energy drop shows an effect equivalent to overaging: The shelf drop recovers by further irradiation in the region of 1 to 4 × 10¹⁹n/cm² for all low copper weld metals except that with 1.7% nickel. The shelf drop sequence of the materials tested is in general similar to ΔT₄₁ sequence with some differences in detail.