The effect of Cu on the kinetics of compound formation between Al and transition metals such as Hf and Ti has been studied by resistivity measurement and Rutherford backscattering (RBS) in the temperature range of 325°C to 450°C. Thin film couples consisting of 150nm-thick transition metal film and 600nm-thick Al-Cu film were used, in which the Cu content was varied from 0 to 9 wt%. The intermetallic phases were found to grow as (time)1/2. Most interestingly, the reaction rate constants were found to decrease significantly with increasing Cu content, but to level off for Cu contents greater than about 2 wt%. The corresponding activation energy for a given Altransition metal reaction was found to increase with increasing Cu for Cu contents up to 2wt%. For example, when 4 wt% Cu is added to Al, the rate constant at 400°C for HfA13 is reduced by about a factor of 5, while the activation energy increases from 1.5 eV to 2.0 eV. RBS results indicate that the Cu addition seems to inhibit the diffusion of Hf into Al. Diffusive intermixing in the Al/Ti system is much smaller and, consequently, the effect of Cu additions in preventing Ti diffusion into the Al is also much smaller.