A two-stage process based on a hydrolytic pretreatment and an alkali extraction was applied to the fractionation of almond shells into cellulose, pentosan and lignin. Autohydrolysis and dilute-acid hydrolysis were studied as pretreatment methods in two different reactor configurations: an isothermal plug-flow reactor and a stirred non-isothermal, batch autoclave. Yield and composition of the pulps resulting from the hydrolytic pretreatment were equivalent for both reactors when compared through the reaction severity concept in this approach, pretreatment severity is quantified by R(OH), a semi-empirical parameter that comprises the effect of temperature, time and acid concentration used during the pretreatment into a single reaction ordinate. Results showed that a pretreatment severity of log(10)R(OH)=4.3 was the optimal to maximize pentosan recovery, with 66% of the potential yield. At this severity, lignin obtained was 64% of the potential yield and 9% of the initial cellulose was degraded. Viscosity average degree of polymerization of the alpha-cellulose remaining in the pulp was reduced to 500 at this severity. Lignin recovery increased with severity to a value of 82% at log(10)R(OH)=4.7, allowing a better separation of the cellulose and the lignin, but also producing a higher depolymerization of the alpha-cellulose. Polydisper sity of the alpha-cellulose also decreased with the severity increase, from 6.3 at log(10)R(OH)=4.0 to a value of 3.4 at log(10)R(OH)=4-7.