Monte Carlo results are presented for a micellar solution where the macroions and small ions are modeled as charged hard spheres embedded in a continuous dielectric. The macroions are charged with 12 negative charges, and the counterions are mono-and/or trivalent cations. The ratio of the diameters of the macroions and counterions is 2.0 nm:0.2 nm. The results for the excess internal energy and osmotic coefficient are presented for three different macroion concentrations, c,, in addition to the structural;information. For c(m) = 0.0005 M and c(m) = 0.001 M the osmotic coefficient presented as a function of the composition parameter x(3) exhibits a small maximum around x(3) = 0.25, while the excess energy is a monotonic function of this parameter. At a higher concentration of macroions, c, = 0.02 M, both the osmotic coefficient and excess internal energy decrease monotonically with increasing value of x(3). Changes in the structure of the solution, brought about by substitution of monovalent counterions with trivalent ones, are reflected in the shape of the macroion-macroion pair-distribution function. The effect of the interaction range was also studied; for this purpose the particles were allowed to interact via the screened Coulomb potential. It is interesting that even for a potential of much shorter range than the original Coulomb potential, i.e., for the screening parameter kappa equal to 0.5 nm(-1), we observe a clustering of equally charged macroions in solution.