Abstract In this work, the reactions involving l-phenylalanine and d, l-tryptophan in the presence of Cu(II) ion were studied. Optimum conditions for the reactions were established as pH 7 and λ = 641 nm. When the reaction was kinetic, it was observed that the following rate formula was found as dA/ A n = k dt and k = 3.2 × 10 −4 s −1, according to absorbance measurements. Using a perpetual change curve, the ratio of [Cu]/[Cu] + l-phenylalanine + [ d, l-tryptophan] was found 1:1:1. According to this result, one molecule of l-phenylalanine and one molecule of d, l-tryptophan react with one molecule Cu(II) ion.