Abstract: Relative permeability of subsurface earthen materials is measured by advancing a probe into the earthen material at a constant rate while simultaneously injecting a fluid sealant/lubricant, or vapor, or liquid, from the probe to the earthen material. The relative permeability of the earthen material is determined by measuring either changes in flow rate or pressure of the injected vapors or fluids. The prescribed method of operation of the probe seals the probe hole allowing collection of discrete vapor samples and also greatly facilitates advancement of the probe.