This work investigates the tracer technique for the measurement of catalyst wetting efficiency, f, in trickle-bed reactor. The model of Ramachandran et al. [1986. A new model for assessment of external liquid-solid contacting in trickle-bed reactors from tracer response measurements. Chemical Engineering Science 41(4), 855-860], based on a 2D description of the tracer diffusion, is applied for the full range of wetting efficiency. It is also extended to account for the effects of axial dispersion, liquid-solid mass transfer, pattern of the wetted zone on the pellet, and distribution of the partial wetting along the reactor. The numerical method for parameter optimization implies a frequency domain least-squares procedure. A sensitivity analysis has been performed proving that the wetting efficiency may be derived with convenient accuracy in usual trickle-bed conditions: high Peclet and Biot numbers. For f > 0.3, it is shown that wetting efficiency can be accurately calculated from apparent particle diffusivities derived in liquid-full conditions (D-eapp,(LF)) and in partial wetting regime (D-eapp,D-TB) using the following relation: f = root D-eapp,D- TB/D-eapp,D- LF .(c) 2007 Elsevier Ltd. All rights reserved.