The casting process variables considered were the shell mould preheat temperature, the pouring temperature and the melt hydrogen content. The paper reports progress on a programme to appraise and optimise the mechanical properties of selected investment castings. The in¯uence of shell preheat temperature, pouring temperature and melt hydrogen content on microporosity and mechanical properties were studied in this paper. Shell preheat temperature and melt hydrogen content are the most important process parameters determining the amount of porosity. All three parameters affect the mechanical properties to varying degrees. q