We have developed a method that uses on-line microdialysis sampling coupled with high-performance liquid chromatography (HPLC) to determine arbutin in whitening cosmetics. The optimum analytical conditions for microdialysis sampling were a probe length of 10mm and a dialysis flow-rate of 5 microl min-1. The accuracy (% bias) for intra-day (n=6) and inter-day (n=30, five consecutive days) analyses ranged from -8.9 to 11.5%, with a precision below 7.64% R.S.D. The calibration curve was linear within the range from 0.1 to 20 mM (R2=0.9989). The detection limit was 15 microM. By comparing the arbutin levels determined this way in the whitening cosmetics with the results obtained from the no-net-flux method, we conclude that our proposed on-line microdialysis-HPLC system displays good accuracy. We evaluated the robustness of our optimum conditions by means of a Plackett-Burman design. Apart from the effect of a low flow-rate of perfusate--an increase of 12.52+/-2.31%--we observed no significant changes in the analyses upon changing the levels of any other parameter. Because this on-line method offers the advantages of simplicity, reliability, the lack of any tedious sample pretreatment process, and a reduced use of organic solvents, we believe that it is suitable for the routine analysis of commercial cosmetics.