The adsorption of methylene blue (MB) onto zirconium phosphate (ZrP) was studied and the adsorption capacity value and isotherms were determined. The adsorption capacity of ZrP was increased after being exposed to gas phase n-butyl amine. The adsorbed MB on ZrP was used as a modifier material in carbon paste electrode (MCPE), which in turn was used in voltammetric investigations and flow injection (FI) amperometric determination of ascorbic acid (AA). A quasi-reversible electrode reaction of adsorbed MB was exhibited. A home made flow-through electrochemical cell with a suitable transparent window for irradiation of the electrode surface was constructed and used for amperometric FI studies. The photoamperometric-FI conditions were optimised as 1.5 mL min(-1) flow rate, a 25 cm transmission tubing length, a 100 mu L injection volume, and a constant applied potential of +100 mV versus SCE. The calibration curve for AA was linear over the concentration range from 1.0 x 10(-6) to 4.0 x 10(-5) M. The relative standard deviation of four replicate injections of 3.0 x 10(-5) M AA was 1.2 %. The results obtained for AA determination in pharmaceutical products are in good agreement with those obtained using the procedure involving the reaction between triiodide and AA. (c) 2005 Elsevier B.V. All rights reserved.