The changes in the chemical constituents of forest residue chips after seven months of storage under different conditions were determined. Two piles, one indoors and the other outdoors, were set up. Each pile comprised different sections where the chips were stored either loose or compacted. Chemical analyses showed that the Klason lignin contents were increased after storage, especially in the areas where high temperatures were measured. The extractives content declined in most of the tested samples. The uronic acid contents of all the forest residue fractions were clearly reduced after indoor storage. Storage conditions in general affected the needle and bark fractions more drastically than the twigs and wood fractions. The chemical modification of the stored material can be attributed to the microbial activity in the pile as well as to the high temperature which developed in the centre of the different sections leading to various chemical transformations.