Organosilicon halides are prepared by heating together an alkyl or an aryl halide and a solid porous contact mass consisting of silicon and copper which is prepared by the process of Specification 575,669, [Group III]. The reaction is preferably carried out at normal pressure and a temperature within the range 200 DEG -500 DEG C., particularly between 250 DEG -400 DEG C. In examples, 1-7 methyl chloride is passed through a tube over the copper silicon alloy at slightly varying conditions of throughput, temperature, and form of catalyst. Example 8 employs ethyl chloride, example 9 uses methyl bromide, example 10 uses ethyl bromide, example 11 uses methyl iodide, example 12 uses methyl fluoride and example 13 uses bromobenzene. Specifications 542,655, 572,740, [Group VIII], and 575,667 also are referred to.