High surface area charcoal bed filters have been used for over a half a century to adsorb undesirable vapors from gas streams. One problem encountered when using these filter beds is that there is presently no simple, reliable, nondestructive method to measure their Residual Adsorption Capacity, RAC. This is particularly critical in situations where harmful vapors are being adsorbed. An investigation has been underway to use pulses of weakly adsorbed gases such as ethane and/or methane to measure RAC. The hypothesis being that these weakly adsorbed gases will “count” unoccupied adsorption sites. In the present study, dimethyl methylphosphonate (DMMP) was used to “irreversibly” occupy available sites to various extents on different filters. The Reduced Retention Time, θ, (the ratio of the adjusted retention time to the space time) and the Resolution (R) between peaks of methane and ethane were found to correlate to RAC under dry and wet (humid) conditions.