We analysed some peculiarities of electromagnetic metamaterials convenient for plasmon-based chemical sensing with enhanced sensitivity. Owing to the vastly extended range of possible values of refractive index, a higher degree of design freedom is enabled in comparison to conventional surface plasmon sensors. After considering some possible approaches to use metamaterials as either surface or bulk adsorbents, we analysed the ultimate performance of such sensing devices which are limited by refractive index fluctuations caused by adsorption and desorption processes. We utilized an approach based on a parallel between the adsorption–desorption mechanism and the carrier generation–recombination in semiconductors. As a special case, we considered structures for the detection of a single gas and used our approach to calculate the spectral density of refractive index fluctuations. We believe that our analysis may be of importance in different practical fields, including e.g. environmental sensing, homeland security and biosensing.