The prepyrolysis of delignified wheat straw is studied. Aqueous alkaline (NaOH-Na2CO3, NaOH-NaHCO3) as well as organosolv alkaline (NaOH-ethanol, NaOH-isopropanol) delignification methods were used. Pretreated straw was thermochemically converted to a solid bio-fuel at 270 degrees for 30 min. Prepyrolysis dry yield and gross caloric value of the material produced were determined to characterize the behaviour of pretreated material during heating and the effects of delignification. The prepyrolytic behaviour of the pretreated material can be described by simple mathematical formulae. In particular, prepyrolysis dry yield is generally a linear function of lignin content, i.e. degree of delignification; moreover, the caloric value of the solid fuel produced after prepyrolysis can also be linearly expressed with lignin content or delignification.