Ten Mannich bases were synthesized in reaction of CH2O or lauryl aldehyde with Me2N(CH)(3)NH2 and p-RC6H4OH (R = Me, OMe, NO2, C12H5) or PhOMe at 45 degrees C optionally under microwave irradn. under lab. conditions. The bases were added to a base diesel oil studied then for homogeneity and interphase tension. Use of microwave heating resulted in a substantial acceleration of the Mannich reaction.