A new theoretical approach for simultaneously determining permeability and porosity, using the pressure-decay data obtained from a probe permeameter, is presented. The approach is based on the general solution for unsteady-state gas flow through the probe tip into a porous half-space. The solution is expressed in terms of pseudopressure and pseudotime. Simple formulae for permeability and porosity are derived. The formulae contain two geometrical factors, G(0) (for permeability) and G(1) (for porosity). Since the pressure-decay data contribute to the formulae through the first two moments of the pseudopressure response function, the procedure of calculating permeability and porosity entails matching those moments. It is shown that the geometrical factor for permeability coincides with that reported in 1988 by Goggin, Thrasher, and Lake for the steady-state problem. Treatment of a small parameter (the ratio of the cube of the inner radius of the tip seal to the gas tank volume) makes it possible to obtain an asymptotic, exponentially decaying expression of the pseudopressure response function, which can be used to estimate the actual probe permeameter response at large times.