The effects of steam explosion pretreatment on the physical and chemical properties of rice straw and on the enzymatic saccharification of the straw were investigated. A wide range of pretreatment conditions were tested. Pretreatment effects were assessed by morphological changes, pore size distribution, pH, soluble sugars concentration, amounts of extracted components, and by enzymatic saccharification of pretreated samples. Rice straw was very finely defiberized by steam explosion. A great increase in pore volume accessible to cellulolytic enzymes was observed during pretreatment at 3.53 MPa for 2 min. On the other hand, pretreatment under very high steam pressure of 4.02 MPa reduced fiber porosity. The steam explosion products were separated into hemicellulose, cellulose, methanol-soluble lignin, and klason lignin. Increasing the steaming time at high steam pressures (temperatures) increased the amount of klason lignin and reduced the amount of hemicellulose, thus emphasizing the need to identify optimal pretreatment conditions for efficient use of biomass. A maximum 83% glucose yield was obtained by enzymatic saccharification of the sample exploded at 3.53 MPa for 2 min. These explosion conditions produce a material that is highly susceptible to attack by cellulolytic enzymes.