Several acid dyes whose molecular sizes were far different were used in this work. They were dyed with nylon fabrics and the characteristics of their rubbing-fastness were investigated. The amount of removed dyes were different from dye to dye and it had nothing to do with molecular size of dye. From kinetic analysis, the removal process of acid dyes could be treated as a reversible reaction of first order. The rate constant of transfer of acid dyes from dyed nylon fabrics to white cloth (k1) and the rate constant of reverse process (k-1) mostly increased with molecular weight of dyes but the equilibrium constant (K) did not depend on it. So far with cotton fabrics, the constants k1 and K generally depended on load weight on white cloth but k-1 did little, while with nylon fabrics dyed with acid dyes all those constants did. The dependence of k1 on load weight was the biggest, k-1 was the second and K was the third.