A catechol sensor based on a polymer-modified graphite electrode has been developed. Poly(N-vinylpyrrolidone) (PNVP) membrane was coated on the surface of a graphite electrode by immobilizing PNVP with gamma radiation in the presence of N-vinylpyrrolidone monomer The response of the sensor to catechol was evaluated by cyclic voltammetry. The anodic peak currents as response signals of the sensor-ere proportional to the concentration of catechol from 10^-5M to 10^-3M. The response time defined as the time required for the current to reach 90% of its maximum value was about 15min. The peak current was enhanced about 2 times compared to the bare graphite electrode. The sensor responds selectively to catechol by 2.2 times higher than ascorbic acid. The origin of enhanced sensitivity and selectivity was evaluated by equilibrium and diffusion dialysis experiments. Partition coefficients of catechol and ascorbic acid between the PNVP-membrane and the aqueous solution were 6.3 and 2.0, respectively. Diffusion coefficients in the PNVP-membrane are 4.2x10^-7 for catechol and 7.0 x10^-7 [cm²/s] for ascorbic acid. Permselectivity through the PNVP-membrane evaluated from the steady state fluxes of catechol and ascorbic acid is ca. 3.0 and it is in good agreement with the voltammetric selectivity. The enhanced response and selectivity of the sensor were discussed with the transport parameters obtained in this work.