Dense phase regions of Zenz-type phase diagrams; i.e., the plot of pressure drop versus superficial gas velocity, with solids mass flowrate and solids loading as parameters, have been quite reasonably predicted for the horizontal pneumatic transport of coarse poly(styrene) and fine PVC particles by the model of Hong et al. Powder Technol., 77 (1993) 107. The definitions of theoretical saltation velocity and critical velocity are given to determine clearly the minimum velocity for fully suspended flow and maximum velocity for plug or extrusion flow. Only the minima in the phase diagram at constant solids mass flowrate for coarse particles are confirmed to coincide with the saltation of the particles, whereas they begin to saltate to the right of the minima in the diagram at a constant solids loading. For the transport of fine particles, saltation occurs at a gas velocity higher than that at the minima in the phase diagrams at both parameters. For both coarse and fine particles, the gas velocities at the minima of phase diagram are recognized to increase with solids mass flowrate and decrease with solids loading respectively. The theoretical critical velocity is found to be smaller than the empirical critical velocity defined as the minimum velocity for the stable strand-type transport.