This work studies the electrodeposition of aluminium–copper alloy on highly pure gold substrates in 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl-sulfonyl) imide ([BMP]Tf2N) ionic liquid. The investigations were carried out using the white viscous upper phase of AlCl3 based [BMP]Tf2N mixture which aluminium can be only deposited. Because of the limited solubility of copper salts in ionic liquids, Cu ions were introduced into the AlCl3–[BMP]Tf2N system by anodic dissolution of the pure Cu sheet. The results reveal that no significant amount of Cu ions can be introduced into the mixture even at elevated temperatures. However, the Cu dissolution in the pure [BMP]Tf2N liquid at 70 °C gave Cu+ ions as calculated from Faraday’s law by weight loss of Cu metal.