An on-line preconcentration method was developed for capillary electrophoresis (CE) with hydride generation-atomic fluorescence spectrometric (HG-AFS) detection of arsenite, arsenate, dimethylarsenic acid, and monomethylarsenic acid. These arsenic species were negatively charged in the sample solution with high pH. When the potential was applied to the electrophoretic capillary, the negatively charged analyte ions moved faster and stacked at the boundary of sample and CE buffer with low pH So, high sample pH in combination with low buffer pH allowed the injection of large sample volumes (similar to 1100 nL). Comparison of the preconcentration of analyte solution, prepared with doubly deionized water and that prepared with lake or river water, indicated that preconcentration was independent on the original matrix. With injection of similar to1100 nL sample, an enrichment factor of 37-50-fold was achieved for the four species. Detection limits for the four arsenic species ranged from 5.0 to 9.3 mug (.) L-1. Precisions (RSDs, n = 5) were in the range of 4.9-6.7% for migration time, 4.7-11 % for peak area, and 4.3-7.1 % for peak height, respectively. The recoveries of the four species in locally collected water solution spiked with 0(.)1 mug-mL(-1) (as As) ranged from 83 to 109%.