Relationships between durability and lithologic characteristics of 61 mudrock samples from North America were investigated with the objective of developing a mudrock-durability classification based on lithologic characteristics. Second-cycle slake-durability index (Id2) was used as a measure of durability, whereas clay content, clay-mineral composition, texture, microfracture frequency, absorption, adsorption, dry density, void ratio, and Atterberg limits were used to characterize mudrock lithology. Based upon the amount of clay-size material, the presence or absence of laminations, and the degree of induration, the mudrocks were subdivided into claystones, mudstones, siltstones, shales, and argillites. The relationships between durability and lithologic characteristics were investigated separately for each class of mudrocks. The results show that different lithologic characteristics best correlate with the durability of each class of mudrocks. The durability of claystones correlates best with the amount of expandable clay minerals and that of mudstones with the frequency of microfractures. The degree of consolidation, as expressed by absorption, influences the durability of both siltstones and shales. The durability of argillites is related to their crystalline texture. Lithologic characteristics can be quantitatively related to a durability classification proposed herein that recognizes three classes of durability: low (Id2 < 50%), medium (Id2 = 50%–85%), and high (Id2 > 85%).Key words: mudrock, durability, lithologic characteristics, classification.