As is well known, solutions of rubber in organic solvents exhibit a variety of noteworthy properties which are characteristic of solutions of lyophilic colloids, to which rubber belongs. These properties, notably high viscosity even in dilute solution and structure viscosity, distinguish these solutions from those of normal substances of low molecular weights. In the present article, the attempt is made to assemble known facts about viscosity phenomena and at the same time to add certain experimental data, heretofore unpublished, to the available literature on the subject.