The mixture of lead tetra-alkyl and an aluminium alkyl formed by electrolysing on a lead anode a compound of the general formula: Me[Al(R1)4] in which Me represents sodium or potassium and R1 represents a straight-chain alkyl radical having 2 to 6 carbon atoms is separated into its components by treating with a compound of the general formula: Me[Al(R1)3OR] in which Me and R1 have the above meanings and R represents an alkyl radical having 2 to 8 carbon atoms or a phenyl group which may be substituted, and then separating the compounds of the mixture. When a mixture of aluminium triethyl and lead tetriethyl formed by electrolysis is treated with an alkali metal alkoxyaluminium triethyl the following reaction takes place: Al(C2H5)3 + Me[Al(OR)(C2H5)3] =Me[Al(C2H5)4] + Al(OR)(C2H5)2