The limitation of sorbents presently used in conservation to buffer changes in relative humidity within an enclosed showcase suggest the need for more efficient buffering materials. Concepts that can be used to estimate the efficiency of different sorbents form the subject of this paper. M values are used to express the buffering capacity of a sorbent within a designated RH range . The hysteresis of a sorbent is accounted for by using a positive value for adsorption and a negative value for desorption. The actual significance of hysteresis on a sorbent within a designated rh range is discussed, and designated M + - values or DM + - values are introduced in order to summarize the actual buffering capacity of a sorbent which exhibits hysteresis within a designated rh range. Four different types of silica gel and Nikkapellet are analyzed to illustrate the use of the above mentioned concepts.