We have demonstrated that the ability of fullerenes to form clusters is essential for the filling of single-walled carbon nanotubes in solution. In solutions where C60 exists in the form of discrete solvated molecules (e.g., in CS2) no fullerene encapsulation in nanotubes takes place, as the large molar excess of solvent compared to solute prohibits C60 from entering the nanotubes. However, in solutions containing large clusters of C60 (e.g., in n-hexane) nanotubes become densely filled with fullerene molecules despite the large excess of solvent. The interactions between carbon nanotubes and fullerene clusters provide an efficient transport of C60 into nanotubes that avoids the detrimental effects of the solvent molecules. This new mechanism provides the first rational explanation of experiments involving nanotube filling with guest-molecules in solution.