The field of the invention is oil sands processing. A solvent treatment system and process for treating a bitumen-containing stream include contacting that stream with a solvent-containing stream to produce an in-line flow of solvent diluted material; supplying the solvent diluted material into a separation vessel with axi-symmetric phase and velocity distribution and/or particular mixing and conditioning features. The solvent addition, mixing and conditioning may be performed with particular Co V, Camp number, co-annular pipeline reactor, pipe wall contact of low viscosity fluid, flow diffusing and/or flow straightening. The processes enable improved performance of downstream unit operations such as separation of high diluted bitumen from solvent diluted tailings.