Desulfurized vacuum residue from petroleum processing was modified by addn. of a vacuum oil and a clarified oil and then treated at 140 degrees C by microwave irradn. (448W for 3 min or 704 W for 2 min). The products were studied for group compn. (asphalthenes, satd. and arom. hydrocarbons, resins) and colloidal stability and inhomogeneity (microscopy). The treatment resulted in an increase in the contents of resins and arom. hydrocarbons as well as in colloidal stability. The efficiency of irradn. increased with the increasing its power.