Mercury (Hg) is a natural, trace component of natural gas. Corrosion of aluminum heat exchangers by liquid metallic Hg can lead to dramatic issues. The quantification of the gaseous Hg concentration in natural gas streams is therefore crucial prior to the implementation of Hg removal units for preventing the formation of liquid Hg. Different methodologies exist for the determination of the Hg concentration in natural gas, one of which relies on the sampling of natural gas at high pressure using stainless-steel cylinders prior to off-site Hg measurement. An inert internal coating is supposed to hamper Hg adsorption, presumably making the Hg analysis reliable. Here, we challenge this statement by showing that even silicon-coated cylinders are inefficient for preventing Hg adsorption on internal walls. Different cylinders were tested for gaseous Hg concentration stability over time in a clean argon matrix. We find that the gaseous Hg concentration sharply declines in almost all tested cylinders (uncoated, polytetrafluoroethylene-coated, and silicon-coated) to reach undetectable levels within a day or two as a result of adsorption, with the notable exception of a brand new silicon-coated cylinder. Heating cylinders up to 190 degrees C allowed for the recovery of most adsorbed Hg and revealed the occurrence of two distinct Hg species with distinct release temperatures. Our results suggest that Hg(0 )is first physically adsorbed and further oxidized, presumably in relation to sulfur compounds covering the internal walls of the cylinders. The newly purchased silicon-coated cylinder kept a constant gaseous Hg concentration over 6 months because it never interacted with any real natural gas sample containing substantial sulfur concentrations relative to Hg.