A method for chemically pretreating - prior to organically coating - a composite metal structure, which includes at least one part made of aluminum and at least one part made of zinc, and optionally a part made of iron, wherein this method: in a first step comprises treating the composite metal structure with a zinc phosphating solution, which on the parts made of zinc and iron results in the formation of a surface-covering crystalline zinc phosphate layer having a coating weight in the range of 0.5 to 5 g/m 2, but creates no zinc phosphate layer having a coating weight of at least 0.5 g/m 2 on the aluminum parts, and subsequently - with or without a water rinsing step in between - in a second step comprises applying a treatment solution having a pH value in the range from 3.5 to 5.5 to the composite metal structure, which on the parts made of zinc and iron removes no more than 50% of the crystalline zinc phosphate deposited in step, but forms a conversion coating on the aluminum parts, which does not represent a surface-covering crystalline phosphate layer having a coating weight of at least 0.5 g/m 2, the zinc phosphating solution in step having a temperature in the range from 20 to 65 °C and containing a quantity of free fluorides (measured in g/l) that is at least 0.005 g/l, but no greater than the quotient from the number 8 and the solution temperature in °C (8/T), the zinc phosphating solution containing at least 0.025 g/l, but less than 1 g/l of silicon in the form of water-soluble inorganic compounds calculated as SiF 6, and the product (Si/mM)x(F/mM) from the concentration of silicon [Si in mM] in the form of water-soluble inorganic compounds and the concentration of free fluoride [F in mM] divided by the point figure of the free acid being no greater than 4.5, wherein the point figure of the free acid in the zinc phosphating solution is at least 0.4 points, but not exceeding a value of 3.0 points. in a first step comprises treating the composite metal structure with a zinc phosphating solution, which on the parts made of zinc and iron results in the formation of a surface-covering crystalline zinc phosphate layer having a coating weight in the range of 0.5 to 5 g/m 2, but creates no zinc phosphate layer having a coating weight of at least 0.5 g/m 2 on the aluminum parts, and subsequently - with or without a water rinsing step in between - in a second step comprises applying a treatment solution having a pH value in the range from 3.5 to 5.5 to the composite metal structure, which on the parts made of zinc and iron removes no more than 50% of the crystalline zinc phosphate deposited in step, but forms a conversion coating on the aluminum parts, which does not represent a surface-covering crystalline phosphate layer having a coating weight of at least 0.5 g/m 2, the zinc phosphating solution in step having a temperature in the range from 20 to 65 °C and containing a quantity of free fluorides (measured in g/l) that is at least 0.005 g/l, but no greater than the quotient from the number 8 and the solution temperature in °C (8/T), the zinc phosphating solution containing at least 0.025 g/l, but less than 1 g/l of silicon in the form of water-soluble inorganic compounds calculated as SiF 6, and the product (Si/mM)x(F/mM) from the concentration of silicon [Si in mM] in the form of water-soluble inorganic compounds and the concentration of free fluoride [F in mM] divided by the point figure of the free acid being no greater than 4.5, wherein the point figure of the free acid in the zinc phosphating solution is at least 0.4 points, but not exceeding a value of 3.0 points.