The lignocellulosic by-products (whole stones and seed husks) obtained from pitted table olive and olive oil processing (respectively) were pretreated under various conditions of steam-explosion, with respect to pressure and time and with a dilute solution of sulfuric acid and without previous acid impregnation. The two materials exhibited considerable difference in behavior. The pre-impregnation of whole stones with low level of acid prior to steam treatment was necessary to improve the fractionation and the autohydrolysis of hemicellulose, while the results with seed husks were similar both with and without an acid catalyst. The lignins recovered from water-washed solid residue by alkaline-extraction followed by acidification were characterized. These lignin preparations from whole stones contained a very high amount of fat and almost no neutral sugars. The results also showed that these lignin preparations were guaiacyl-syringyl lignin, similar to exploded hardwood lignin, with an extensive cleavage of the beta-aryl-ether linkage. The effect of treatment severity on the major physical properties of cellulose was also investigated. The cellulose fraction derived from steam-exploded whole stones was rapidly depolymerized as severity increased, leading to a degree of polymerization of 180, at a severity of log R-0 = 4.07. In seed husks, under similar severity conditions, depolymerization of cellulose was much slower, and only reached a value of 821 (at log R-0 = 4.07) or a value of 600 at log R-0 = 4.34 without acid impregnation. The cellulose derived from both steam-exploded materials was moderately crystalline, the relative crystallinity index values of seed husks being higher than those of whole stones. The results indicated that the cellulose from seed husks was more protected than that of the whole stones in the steam-explosion process, and could be used to produce a cellulose with different properties and applications depending on severity and on the different treated material.