Neutron reflection has been used to measure the dynamic surface concentration, Gamma(dyn), of the cationic surfactant CTAB in an overflowing cylinder (OFC). This is the first direct measurement of Gamma(dyn) for a surfactant at an expanding liquid surface. Gamma(dyn) was measured for bulk concentrations between 1.8 and 0.125 mM, corresponding to areas per molecule of 42 to > 300 Angstrom(2). Careful account had to be taken of the curvature of the surface in establishing calibration procedures. Gamma(dyn) was less than the equilibrium surface concentration Gamma(e) at all concentrations below the cmc but approached Gamma(e) at bulk concentrations well above the cmc, as expected for an expanding surface. Ellipsometry has been used previously to measure the coefficient of ellipticity of surfactant solutions in an OFC. The neutron reflection data were used to calibrate the ellipticity of the surface in terms of Gamma(dyn) and ellipsometry was then used to map changes in Gamma(dyn) across the surface of the OFC with a precision of 2 x 10(-8) mol m(-2). The ellipticity can also be calibrated against Gamma(e) for static solutions, and this calibration curve can be used to calculate Gamma(dyn) from the dynamic ellipticities. The values of Gamma(dyn) determined by this indirect route were in excellent agreement with the direct measurements by neutron reflection.