A metallographic study was made of the diffusion zones present in copper‐&agr;‐brass, copper‐nickel, and gold‐silver diffusion couples. Usual sandwich type couples and also vapor‐solid type couples in which the higher vapor pressure component was diffused either into or out of the couple through the vapor phase were investigated. Significant porosity was always generated by diffusion in the sandwich couples and also in the vapor‐solid couples when the higher vapor pressure component was diffused out of solid solution. Upon inward diffusion from the vapor porosity formed in the gold‐silver couples but not in the copper‐&agr;‐brass or copper‐nickel couples.It is concluded that porosity may form in any alloy system when the components diffuse at different rates and that it may cause appreciable error in the measurement of diffusion coefficients if ignored.