In dyeing PET/Spandex blends with disperse dyes, many disperse dyes are absorbed more readily into Spandex than PET. Inorganic/organic(I/O) values can characterize the affinity of disperse dyes for Spandex and PET fibers. As I/O values of disperse dyes approach that of Spandex, the dye uptake in Spandex increases. On the contrary, the disperse dyes of higher I/O values are absorbed more into PET than Spandex. The dye uptake in PET increases with increasing I/O values of disperse dyes. As I/O values of disperse dyes increase, the rate of dyeing become slower in the dyeing of PET/Spandex blends. Hence, the affinity and the compatibility of disperse dyes in PET/Spandex blends dyeing can be evaluated with I/O value.