Measurements were made of the thermal resistance of porous deposits of various thicknesses under both single-phase forced convection and flow-bailing conditions. Both synthetic deposits and deposits on tubes removed from operating steam generators were used in this investigation. The thermal resistance was modeled as the sum of two components: one associated with conduction through the porous deposit and a second associated with the effect of surface roughness. The conductive component of the thermal resistance was always positive, whereas surface roughness made a negative contribution to the thermal resistance; i.e., roughness enhanced the rate of heat transfer. Thermal conductivity of the porous deposits was higher for single-phase forced convection, whereas the effect of deposit roughness on thermal resistance was higher under flaw-boiling conditions. The results are discussed in terms of the effect of composition, morphology, surface roughness, and the mode of heat transfer on the thermal resistance of porous deposit.