The hydrodynamic behavior of paper pulp slurries in vertical laminar flow has been observed and modeled. Radiotracers were used to tag both the liquid and solid phases of slurries flowing in 4 and 30 cm pipes. Residence time distributions were determined for each phase and were used to derive and compare several flow models. A model consisting of a central core in plug flow circumscribed by a clear Newtonian annulus provides a good representation of the observed slurry behavior. Correlations between the model parameters and the Reynolds number and slurry consistency are presented, and implications of the results on the design of slurry flow reactors are discussed.