An isolation technology for radio frequency (rf) applications based on unoxidized porous Si (PS) is demonstrated. This study examines all the important issues pertinent to incorporating PS with Si very-large-scale integration (VLSI) technology, where PS is used as a semi-insulating material. Specifically, the issues on rf isolation performance of PS as a function of porosity [from coplanar waveguide (CPW) line measurements] and PS thickness (from on-chip inductors) and the stress generated from incorporating PS regions by anodization are discussed in detail. CPW line measurements show that the relative dielectric constant of PS films decreases from 9 to 3 with increasing porosity from 24% to 78%. PS is a very low loss material with loss tangent <0.001 at 20 GHz when its porosity is above 51%. rf crosstalk through a Si substrate can be reduced to that through air by inserting a PS trench between noise generating circuit and noise sensing circuit. On-chip spiral inductors fabricated on top of PS regions of through-the-wafer thickness have Qmax of about 29 at 7 GHz and resonant frequency higher than 20 GHz. With the additional advantage of planar topography and mechanical integrity, we show that unoxidized PS is an outstanding material for rf isolation in Si VLSI.