tA solid phase extraction using multiwalled carbon nanotubes, modified with a tertiary amine, ALIQUAT336, has been developed as a preconcentration technique for the determination of trace amounts ofAs(V) in water samples. After “batch-based mode” As(V) adsorption on ALIQUAT 336-Multiwalled carbonnanotubes, determinations were made directly on the solid by X-Ray Fluorescence spectrometry, whichreduces the number of reagents used and minimizes the sample handling.The method herein described combines the advantages of solid phase extraction with the benefits ofany X-Ray Fluorescence method associated to the large areas held by the carbon nanotubes, which makethem a promising solid sorbent for preconcentration procedures.The enrichment factor was calculated considering that the thin film obtained from the 100 mL solutionof 1000 g L-1As had a final thickness of 0.04 mm and a final diameter of 16.7 mm, the volume depositedon the pellet was 0.0088 cm3. Thus the preconcentration factor obtained was 104fold, which representeda highly satisfactory value for trace analysis by X-Ray Fluorescence.Satisfactory results were obtained for the determination of arsenic in samples of the standard referencematerial “NIST 1643e—Trace Elements in Water and drinking water”.