A new class of colloidal dielectric fluids was formulated with the objective of enhancing the dielectric and cooling performance of the constitutive base fluids a mineral and a synthetic oil for application in electro thermal environment. Colloidal particles with different dielectric properties were suspended by means of surfactants and sonication. Dielectric strength and thermal conductivity measurements were carried out for a number of different colloidal systems with varying particle volume fraction. An alternative non-destructive breakdown testing approach that limits the amount of electrical energy transferred to the test sample was employed to characterize the dielectric properties of suspensions. In general the addition of particles to the base fluids was detrimental for the overall dielectric strength except when iron oxide nanoparticles were added to mineral oil. On the other hand the thermal conductivity of the base fluids was increased by suspending the particles. Dielectric strength and thermal conductivity measurements of the different colloidal systems are interpreted and compared in light of the physical properties of the suspended particles and their base fluids. © 2008 Elsevier B.V. All rights reserved.