A residence time distribution (RTD) model to describe the liquid trickle flow in a trickle-bed reactor packed with porous particles and operated both under partially and fully wetted conditions was proposed based on a simple representation of the liquid flow structure. The model views the external liquid stream as divided into a dynamic zone where an axially dispersed plug flow pattern prevails, and an external stagnant (or static) zone contiguous to both the dynamic zone and the partially wetted porous particles. The model incorporates mass transfer between (i) external dynamic and stagnant zones, (ii) dynamic tone and nearby partially wetted porous particles, (iii) stagnant zone and adjacent partially wetted particles, and (iv) finally intraparticle diffusion. The model parameters were derived from liquid residence time distribution tests with various air/Newtonian and air/non-Newtonian systems. Analysis of the dynamic tracer impulse-response data of the liquid revealed the significance of the mass transfer resistance between static liquid and adjacent wetted particles, intraparticle diffusion resistance, as well as partial wetting. By properly accounting for intraparticle diffusion, peculiarly high liquid axial dispersion coefficients were obtained for low liquid velocities and high carboxymethyl-cellulose (CMC) concentrations. Finally, the deficiency of lumping static liquid-solid mass transfer, internal diffusion, and partial wetting in the Peclet number and the number of transfer units was discussed.